BARCELONA, Aug 18 (Reuters) - Spain mounted a sweeping anti-terror operation on Friday after a suspected Islamist militant drove a van into crowds in Barcelona, killing 13 people before fleeing, in what police suspect was one of multiple planned attacks.

Islamic State claimed responsibility for the deadly rampage along the city's most famous avenue on Thursday, which was packed with tourists taking an afternoon stroll. The death toll could rise, with more than 100 injured, authorities said.

As security forces hunted for the van's driver, who was seen escaping on foot, police said they had killed five attackers on Thursday night in Cambrils, a town south of Barcelona, to thwart a "terrorist attack" using explosive belts. Six civilians and a police officer were injured in the operation.

Click through reactions and images from the scene:



35 PHOTOS Van crashes into crowd in Barcelona See Gallery Van crashes into crowd in Barcelona People react as they leave a cordoned off area after a van ploughed into the crowd, killing two persons and injuring several others on the Rambla in Barcelona on August 17, 2017. A driver deliberately rammed a van into a crowd on Barcelona's most popular street on August 17, 2017 killing at least two people before fleeing to a nearby bar, police said. Officers in Spain's second-largest city said the ramming on Las Ramblas was a 'terrorist attack' and a police source said one suspect had left the scene and was 'holed up in a bar'. The police source said they were hunting for a total of two suspects. / AFP PHOTO / Josep LAGO (Photo credit should read JOSEP LAGO/AFP/Getty Images) Policemen walk on the Rambla after a van ploughed into the crowd, killing two persons and injuring several others on the Rambla in Barcelona on August 17, 2017. A driver deliberately rammed a van into a crowd on Barcelona's most popular street on August 17, 2017 killing at least two people before fleeing to a nearby bar, police said. Officers in Spain's second-largest city said the ramming on Las Ramblas was a 'terrorist attack' and a police source said one suspect had left the scene and was 'holed up in a bar'. The police source said they were hunting for a total of two suspects. / AFP PHOTO / Josep LAGO (Photo credit should read JOSEP LAGO/AFP/Getty Images) Policemen and medical staff members stand in the entrance of a mall after a van ploughed into the crowd, killing two persons and injuring several others on the Rambla in Barcelona on August 17, 2017. A driver deliberately rammed a van into a crowd on Barcelona's most popular street on August 17, 2017 killing at least two people before fleeing to a nearby bar, police said. Officers in Spain's second-largest city said the ramming on Las Ramblas was a 'terrorist attack' and a police source said one suspect had left the scene and was 'holed up in a bar'. The police source said they were hunting for a total of two suspects. / AFP PHOTO / LLUIS GENE (Photo credit should read LLUIS GENE/AFP/Getty Images) An elderly woman accompanied by young people reacts after a van ploughed into the crowd, killing two persons and injuring several others on the Rambla in Barcelona on August 17, 2017. A driver deliberately rammed a van into a crowd on Barcelona's most popular street on August 17, 2017 killing at least two people before fleeing to a nearby bar, police said. Officers in Spain's second-largest city said the ramming on Las Ramblas was a 'terrorist attack' and a police source said one suspect had left the scene and was 'holed up in a bar'. The police source said they were hunting for a total of two suspects. / AFP PHOTO / Josep LAGO (Photo credit should read JOSEP LAGO/AFP/Getty Images) A woman gestures as she is escorted out by Spanish policemen outside a cordoned off area after a van ploughed into the crowd, killing 13 persons and injuring over 80 on the Rambla in Barcelona on August 17, 2017. A driver deliberately rammed a van into a crowd on Barcelona's most popular street on August 17, 2017 killing at least 13 people before fleeing to a nearby bar, police said. Officers in Spain's second-largest city said the ramming on Las Ramblas was a 'terrorist attack'. / AFP PHOTO / PAU BARRENA (Photo credit should read PAU BARRENA/AFP/Getty Images) Spanish policemen accompany people leaving the cordoned off area after a van ploughed into the crowd, killing 13 persons and injuring over 50 on the Rambla in Barcelona on August 17, 2017. A driver deliberately rammed a van into a crowd on Barcelona's most popular street on August 17, 2017 killing at least 13 people before fleeing to a nearby bar, police said. Officers in Spain's second-largest city said the ramming on Las Ramblas was a 'terrorist attack' and a police source said one suspect had left the scene and was 'holed up in a bar'. The police source said they were hunting for a total of two suspects. / AFP PHOTO / LLUIS GENE (Photo credit should read LLUIS GENE/AFP/Getty Images) BARCELONA, SPAIN - AUGUST 17: Paramedics are seen near to the scene of a terrorist attack in the Las Ramblas area on August 17, 2017 in Barcelona, Spain. Officials say 13 people are confirmed dead and at least 50 injured after a van plowed into people in the Las Ramblas area of the city this afternoon. (Photo by David Ramos/Getty Images) BARCELONA, SPAIN - AUGUST 17: Police officers take security measures at the area after a van plowed into the crowd, injuring several people in Barcelona, Spain on August 17, 2017. (Photo by Albert Llop/Anadolu Agency/Getty Images) A woman cries as she and a man are escorted by Spanish policemen outside a cordoned off area after a van ploughed into the crowd, killing 13 persons and injuring over 80 on the Rambla in Barcelona on August 17, 2017. A driver deliberately rammed a van into a crowd on Barcelona's most popular street on August 17, 2017 killing at least 13 people before fleeing to a nearby bar, police said. Officers in Spain's second-largest city said the ramming on Las Ramblas was a 'terrorist attack'. / AFP PHOTO / PAU BARRENA (Photo credit should read PAU BARRENA/AFP/Getty Images) ATTENTION EDITORS - VISUALS COVERAGE OF SCENES OF DEATH AND INJURY: People attend to injured persons at the scene after a van crashed into pedestrians near the Las Ramblas avenue in central Barcelona, Spain August 17, 2017. REUTERS/Giselle Loots SPAIN OUT. MANDATORY CREDIT. ATTENTION EDITORS - VISUALS COVERAGE OF SCENES OF DEATH AND INJURY: People attend to injured persons at the scene after a van crashed into pedestrians near the Las Ramblas avenue in central Barcelona, Spain August 17, 2017. REUTERS/Giselle Loots SPAIN OUT. MANDATORY CREDIT. TPX IMAGES OF THE DAY ATTENTION EDITORS - VISUALS COVERAGE OF SCENES OF DEATH AND INJURY: A dead person is seen at the scene after a van crashed into pedestrians near the Las Ramblas avenue in central Barcelona, Spain August 17, 2017. REUTERS/Giselle Loots MANDATORY CREDIT. SPAIN OUT. Two young girls run to leave a cordoned off area after a van ploughed into the crowd, killing two persons and injuring several others on the Rambla in Barcelona on August 17, 2017. A driver deliberately rammed a van into a crowd on Barcelona's most popular street on August 17, 2017 killing at least two people before fleeing to a nearby bar, police said. Officers in Spain's second-largest city said the ramming on Las Ramblas was a 'terrorist attack' and a police source said one suspect had left the scene and was 'holed up in a bar'. The police source said they were hunting for a total of two suspects. / AFP PHOTO / LLUIS GENE (Photo credit should read LLUIS GENE/AFP/Getty Images) A woman cries as she phones after a van ploughed into the crowd, killing one person and injuring several others on the Rambla in Barcelona on August 17, 2017. Police in Barcelona said they were dealing with a 'terrorist attack' after a vehicle ploughed into a crowd of pedestrians on the city's famous Las Ramblas boulevard on August 17, 2017. Police were clearing the area after the incident, which has left a number of people injured. / AFP PHOTO / Pau Barrena (Photo credit should read PAU BARRENA/AFP/Getty Images) TOPSHOT - People leave a cordoned off area after a van ploughed into the crowd, killing one person and injuring several others on the Rambla in Barcelona on August 17, 2017. Police in Barcelona said they were dealing with a 'terrorist attack' after a vehicle ploughed into a crowd of pedestrians on the city's famous Las Ramblas boulevard on August 17, 2017. Police were clearing the area after the incident, which has left a number of people injured. / AFP PHOTO / Pau Barrena (Photo credit should read PAU BARRENA/AFP/Getty Images) A still image from video shows a police officer gesturing while walking across a road, after a van crashed into people in the centre of Barcelona, Spain, August 17, 2017. REUTERS TV via REUTERS A still image from video shows a police officer ushering people away on a street, after a van crashed into people in the centre of Barcelona, Spain, August 17, 2017. REUTERS TV via REUTERS A still image from video shows a police cordon on a street in Barcelona, Spain following a van crash August 17, 2017. REUTERS TV via REUTERS Abandoned air balloons of a street vendor tied to a chair fly at the empty Plaza de Catalunya after a van ploughed into the crowd, injuring several persons on the Rambla in Barcelona on August 17, 2017. Police in Barcelona said they were dealing with a 'terrorist attack' after a vehicle ploughed into a crowd of pedestrians on the city's famous Las Ramblas boulevard on August 17, 2017. Police were clearing the area after the incident, which has left a number of people injured. / AFP PHOTO / Josep LAGO (Photo credit should read JOSEP LAGO/AFP/Getty Images) BARCELONA, SPAIN - AUGUST 17: People move away from the area after a van plowed into the crowd, injuring several people in Barcelona, Spain on August 17, 2017. (Photo by Albert Llop/Anadolu Agency/Getty Images) A person is stretched out of a mall by medical staff members in a cordoned off area after a van ploughed into the crowd, injuring several persons on the Rambla in Barcelona on August 17, 2017. Police in Barcelona said they were dealing with a 'terrorist attack' after a vehicle ploughed into a crowd of pedestrians on the city's famous Las Ramblas boulevard on August 17, 2017. Police were clearing the area after the incident, which has left a number of people injured. / AFP PHOTO / Josep LAGO (Photo credit should read JOSEP LAGO/AFP/Getty Images) Policemen stand next to vehicles in a cordoned off area after a van ploughed into the crowd, injuring several persons on the Rambla in Barcelona on August 17, 2017. / AFP PHOTO / Josep LAGO (Photo credit should read JOSEP LAGO/AFP/Getty Images) A policemen stand as he blocks the street to a cordoned off area after a van ploughed into the crowd, injuring several persons on the Rambla in Barcelona on August 17, 2017. / AFP PHOTO / PAU BARRENA (Photo credit should read PAU BARRENA/AFP/Getty Images) Firefighters stands outside an evacuated mall after a van ploughed into the crowd, injuring several persons on the Rambla in Barcelona on August 17, 2017. / AFP PHOTO / Josep LAGO (Photo credit should read JOSEP LAGO/AFP/Getty Images) Medical staff members and policemen stand in a cordoned off area after a van ploughed into the crowd, injuring several persons on the Rambla in Barcelona on August 17, 2017. Police in Barcelona said they were dealing with a 'terrorist attack' after a vehicle ploughed into a crowd of pedestrians on the city's famous Las Ramblas boulevard on August 17, 2017. Police were clearing the area after the incident, which has left a number of people injured. / AFP PHOTO / Josep LAGO (Photo credit should read JOSEP LAGO/AFP/Getty Images) A policeman stands next to an ambulance after a van ploughed into the crowd, injuring several persons on the Rambla in Barcelona on August 17, 2017. / AFP PHOTO / Josep LAGO (Photo credit should read JOSEP LAGO/AFP/Getty Images) Plain-clothes policemen phone as they walk past police cars in a cordoned off area after a van ploughed into the crowd, injuring several persons on the Rambla in Barcelona on August 17, 2017. / AFP PHOTO / Josep LAGO (Photo credit should read JOSEP LAGO/AFP/Getty Images) A policemen and a medical staff member stand past police cars and an ambulance in a cordoned off area after a van ploughed into the crowd, injuring several persons on the Rambla in Barcelona on August 17, 2017. / AFP PHOTO / Josep LAGO (Photo credit should read JOSEP LAGO/AFP/Getty Images) A person is stretched out of a mall by medical staff members in a cordoned off area after a van ploughed into the crowd, injuring several persons on the Rambla in Barcelona on August 17, 2017. Police in Barcelona said they were dealing with a 'terrorist attack' after a vehicle ploughed into a crowd of pedestrians on the city's famous Las Ramblas boulevard on August 17, 2017. Police were clearing the area after the incident, which has left a number of people injured. / AFP PHOTO / Josep LAGO (Photo credit should read JOSEP LAGO/AFP/Getty Images) Armed policecen stand in a cordoned off area after a van ploughed into the crowd, injuring several persons on the Rambla in Barcelona on August 17, 2017. Police in Barcelona said they were dealing with a 'terrorist attack' after a vehicle ploughed into a crowd of pedestrians on the city's famous Las Ramblas boulevard on August 17, 2017. Police were clearing the area after the incident, which has left a number of people injured. / AFP PHOTO / Josep LAGO (Photo credit should read JOSEP LAGO/AFP/Getty Images) An armed policeman stands behind a tree in a cordoned off area after a van ploughed into the crowd, injuring several persons on the Rambla in Barcelona on August 17, 2017. Police in Barcelona said they were dealing with a 'terrorist attack' after a vehicle ploughed into a crowd of pedestrians on the city's famous Las Ramblas boulevard on August 17, 2017. Police were clearing the area after the incident, which has left a number of people injured. / AFP PHOTO / Josep LAGO (Photo credit should read JOSEP LAGO/AFP/Getty Images) Armed policemen arrive in a cordoned off area after a van ploughed into the crowd, injuring several persons on the Rambla in Barcelona on August 17, 2017. Police in Barcelona said they were dealing with a 'terrorist attack' after a vehicle ploughed into a crowd of pedestrians on the city's famous Las Ramblas boulevard on August 17, 2017. Police were clearing the area after the incident, which has left a number of people injured. / AFP PHOTO / Josep LAGO (Photo credit should read JOSEP LAGO/AFP/Getty Images) BARCELONA, SPAIN - AUGUST 17: A police stands as he blocks the traffic after a van plowed into the crowd, injuring several people in Barcelona, Spain on August 17, 2017. (Photo by Albert Llop/Anadolu Agency/Getty Images) Pigeons gather on the empty Plaza de Catalunya after a van ploughed into the crowd, injuring several persons on the Rambla in Barcelona on August 17, 2017. Police in Barcelona said they were dealing with a 'terrorist attack' after a vehicle ploughed into a crowd of pedestrians on the city's famous Las Ramblas boulevard on August 17, 2017. Police were clearing the area after the incident, which has left a number of people injured. / AFP PHOTO / Josep LAGO (Photo credit should read JOSEP LAGO/AFP/Getty Images) Up Next See Gallery Discover More Like This HIDE CAPTION SHOW CAPTION of SEE ALL BACK TO SLIDE

Police said the Cambrils plot was linked to the van attack.

Before the van plowed into the tree-lined walkway of Las Ramblas, one person was killed in an explosion in a house in a separate town southwest of Barcelona, police said. Residents there were preparing explosives, a police source added.

Police said they had arrested two men, a Moroccan and a man from Spain's north African enclave of Melilla, though neither was the van driver. They added that the situation in Cambrils was under control.

It was still not clear how many people had been involved in the van attack and other incidents on Thursday.

Witnesses to the van attack said the white vehicle had zigzagged at high speed down Las Ramblas, ramming pedestrians and cyclists, sending some hurtling through the air and leaving bodies strewn in its wake.

Islamic State's Amaq news agency said: "The perpetrators of the Barcelona attack are soldiers of the Islamic State and carried out the operation in response to calls for targeting coalition states" - a reference to a U.S.-led coalition against the Sunni militant group.

Spain has several hundred soldiers in Iraq providing training to local forces in the fight against Islamic State, but they are not involved in ground operations.

The Islamic State claim could not immediately be verified.

If the involvement of Islamist militants is confirmed, it would be the latest in a string of attacks in the past 13 months in which they have used vehicles to bring carnage to the streets of European cities.

That modus operandi - crude, deadly and very hard to prevent - has killed well over 100 people in Nice, Berlin, London and Stockholm.

British tourist Keith Welling, who arrived in Barcelona on Wednesday with his wife and 9-year-old daughter, said they saw the van drive past them down the avenue and took refuge in a restaurant when panic broke out and the crowd started running.

"People were shouting and we heard a bang and someone cried that it was a gunshot ... Me and my family ran into the restaurant along with around 40 other people.

"At first people were going crazy in there, lots of people crying, including a little girl around three years old."

It was the deadliest attack in Spain since March 2004, when Islamist militants placed bombs on commuter trains in Madrid, killing 191 people and wounding more than 1,800.

Spanish Prime Minister Mariano Rajoy announced three days of official mourning for what he called a "jihadist attack."

The Spanish royal household said on Twitter: “They are murderers, nothing more than criminals who are not going to terrorize us. All of Spain is Barcelona.”

U.S. President Donald Trump said: "The United States condemns the terror attack in Barcelona, Spain, and will do whatever is necessary to help."

He added: "Be tough & strong, we love you!"

BODIES ON THE GROUND

Police said the two men detained on Thursday had been arrested in two towns, Ripoll and Alcanar, both in the region of Catalonia, of which Barcelona is the capital.

The explosion was also in the town of Alcanar, in the early hours of Thursday. One person died and another was injured in that incident, police said.

Police said they also shot dead on Thursday a man who had driven a car into a police checkpoint in Barcelona, though they had no evidence he was connected with the van attack.

Mobile phone footage showed several bodies strewn along the Ramblas, some motionless. Paramedics and bystanders bent over them, treating them and trying to comfort those still conscious.

Around them, the boulevard was deserted, covered in rubbish and abandoned objects including hats, flip-flops and a pram.

Belgium's foreign minister said a Belgian was among the dead.

Regional head Carles Puigdemont said people had been flocking to hospitals in Barcelona to give blood.

Susana Elvira Carolina, 33, who works at a shop on Las Ramblas, had just entered her building when the van struck.

"We had a window and you could see the bodies lying from there, you could see how people were run over ... We were shutting down the blinds but people kept coming in and we had to keep it open so they could enter the shop."

TOURIST DRAW

The incident took place at the height of the tourist season in Barcelona, which is one of Europe's top travel destinations with at least 11 million visitors a year.

French President Emmanuel Macron, whose nation has suffered some of Europe's deadliest militant attacks in recent years, tweeted: "All my thoughts and France's solidarity to the victims of the tragic attack in Barcelona."

Authorities in Vic, a small town outside Barcelona, said a van had been found there in connection with the attack. Spanish media had earlier reported that a second van had been hired as a getaway vehicle.

Barcelona is the capital of the wealthy northeastern region of Catalonia, which plans to hold a popular vote on Oct. 1 on whether it should secede from Spain. The central government says the vote cannot go ahead because it is unconstitutional.

Before Thursday's attack, government data showed that police had arrested 11 suspected jihadists in the Barcelona area so far this year, more than anywhere else in Spain.

(Reporting by Angus Berwick, Sarah White, Julien Toyer and Madrid newsroom; Alissa de Carbonnel in Belgium; Ali Abdelaty and Ahmed Aboulenein in Cairo; Writing by Mark Trevelyan; Editing by Mark Bendeich, Nick Tattersall and Lisa Shumaker)