(CNN) Spain saw one of its most violent days in recent memory as a spate of incidents throughout the country appeared to be connected to a terror attack Thursday in Barcelona that left 13 people dead and more than 100 injured.

Authorities said they are working under the assumption that two other deadly events , a terrorist incident in the seaside city of Cambrils and a house explosion farther down the coast in Alcanar, were linked to the van attack in Barcelona that had ISIS taking credit.

Also Thursday, two police officers in Barcelona were hurt when they were hit by a car, but police were unsure whether that was related to the other incidents.

The deadly events began in the early evening with a van plowing through crowds on the renowned Las Ramblas avenue, a popular tourist section of Barcelona. Authorities said of the 80 people taken to hospitals, 15 were seriously hurt.

As police searched for the van driver, Spain's Prime Minister called it an act of "jihadi terrorism."

Here are the latest developments in a tragic day:

• Two suspects -- one from Morocco, one from the Spanish enclave of Melilla -- were arrested in connection with the Barcelona attack, Catalan Police Chief Josep Lluis Trapero said.

• One suspect in the Barcelona attack is on the run. "The driver abandoned the van and escaped from the area," Trapero said.

• About 115 kilometers to the southwest, there was a second attack early Friday. Catalan police tweeted that five suspected terrorists were killed in Cambrils. Emergency officials said six civilians and a police officer were injured.

• One person was killed in an explosion at a house in Alcanar, around 200 kilometers (120 miles) southwest of Barcelona.

• Catalan police said early Friday they are "working under the hypothesis that the terrorists taken down in Cambrils were related to the events that took place in Barcelona and Alcanar."

• A driver ran over two police officers at a security checkpoint in Barcelona, police said, and the driver was found near the city. The two officers suffered minor injuries and did not need hospital treatment, police said. It was unclear whether that incident was related to the terror attack.

• ISIS' media wing, Amaq, said the perpetrators of the Barcelona attack were "soldiers of the Islamic State." However, ISIS has not explicitly claimed responsibility.

The Barcelona attack was one of the most deadly in Spain since more than 190 people were killed in a March 2004 attack against commuter trains

Photos: Terror attacks in Spain A makeshift memorial pays tribute to those who were killed in a terror attack in Barcelona, Spain, on Thursday, August 17. A van rammed into a crowd of people near the popular tourist area of Las Ramblas. Early the next morning, a group of five attackers drove into pedestrians in the Spanish town of Cambrils, about 75 miles south of Barcelona. Hide Caption 1 of 19 Photos: Terror attacks in Spain Two women cry at the street memorial in Barcelona on Friday, August 18. Hide Caption 2 of 19 Photos: Terror attacks in Spain A man embraces a police officer in Cambrils on August 18. Earlier in the day, officers had engaged in a shootout with five attackers who drove a car into several pedestrians. All five were shot dead by police, four of them by one officer, police said. Hide Caption 3 of 19 Photos: Terror attacks in Spain Pigeons fly over a Barcelona crowd that gathered for a minute of silence on August 18. Hide Caption 4 of 19 Photos: Terror attacks in Spain Spain's King Felipe VI joins other officials in observing a minute of silence in Barcelona's Plaça de Catalunya. Hide Caption 5 of 19 Photos: Terror attacks in Spain Police officers investigate a car in Cambrils. Hide Caption 6 of 19 Photos: Terror attacks in Spain The van that plowed into the crowd in Barcelona is towed away from Las Ramblas on August 18. Hide Caption 7 of 19 Photos: Terror attacks in Spain Police officers check the area after towing away the van. Hide Caption 8 of 19 Photos: Terror attacks in Spain Police accompany clients of a store outside a cordoned off-area of Barcelona. Las Ramblas is especially crowded in the summer, the height of tourist season. The promenade passes by kiosks, flower sellers, cafes and bars. Hide Caption 9 of 19 Photos: Terror attacks in Spain Police officers work at the scene in Cambrils. Hide Caption 10 of 19 Photos: Terror attacks in Spain Police check people's identities as they clear Las Ramblas on August 17. Hide Caption 11 of 19 Photos: Terror attacks in Spain People evacuate an area of Barcelona on August 17. Hide Caption 12 of 19 Photos: Terror attacks in Spain Medics and police tend to injured people near the scene of the attack in Barcelona. Hide Caption 13 of 19 Photos: Terror attacks in Spain A woman cries as she speaks on her phone in Barcelona on August 17. Hide Caption 14 of 19 Photos: Terror attacks in Spain People react after the incident in Barcelona. Hide Caption 15 of 19 Photos: Terror attacks in Spain A police officer asks people to move back in Barcelona. Hide Caption 16 of 19 Photos: Terror attacks in Spain An injured person is carried by police in Barcelona. Hide Caption 17 of 19 Photos: Terror attacks in Spain People flee the scene after the attack at Las Ramblas. Hide Caption 18 of 19 Photos: Terror attacks in Spain The Catalan emergency services urged people via Twitter to avoid going out or undertaking any other type of movement that is not "strictly necessary." Hide Caption 19 of 19

Terror in the streets of another European city

It was the latest in a series of attacks in Europe in which vehicles have been used to mow down pedestrians in public spaces. More than 100 people have died in similar attacks in Berlin, London and Nice.

Reports of the incident emerged on social media about 5 p.m. (11 a.m. ET). Photographs and videos showed people fleeing the area. About two hours later, police confirmed a terror attack.

As the incident unfolded, police told everyone in the vicinity of Plaça de Catalunya and Las Ramblas to remain indoors until told it was safe to go outside. Footage posted to social media by witnesses showed chaotic scenes with people lying in the street, apparently dead or injured.

Information about most of the victims has not been released, but one Belgian was killed in the attack, Belgian Foreign Ministry spokesman Jose de Pierpont said.

Spanish Prime Minister Mariano Rajoy spoke of his grief and offered condolences to the families of the victims.

"I want to also express my solidarity with all of Spain to the city of Barcelona, today hit by jihadi terrorism, like other cities have been in the world," Rajoy said.

Witness reports gunshots

One witness told local media the situation was "very tense" and that all surrounding shops were evacuated. The witness said at least eight ambulances were at the scene. Emergency services said the area had been cordoned off and all public transportation stopped.

Another witness who was hiding in a shop nearby heard gunshots, according to state-run broadcaster TVE24. A third said he saw a van driving "around 80" kph, or 50 mph. He said "there is no doubt it was intentional," according to TVE24.

Ali Shirazinia, who was cycling alongside Las Ramblas at the time, told CNN he heard "a lot of screams" and saw the crowds split along the busy promenade.

Then he heard what sounded like the driver flooring the accelerator and saw a white van with blue markings come hurtling down the street. "It literally came right down the Ramblas and ran into people on every side," he said.

"The Ramblas is full of pedestrians, street merchants, street performers, and I saw people flying into the air and everyone was running into the shops on either side of the Ramblas, a lot of people were shocked."

Tourist Susan McClean told CNN she saw a "tidal wave" of people running away from Las Ramblas after the incident.

She ducked into a nearby shop and the shutters were pulled down while police sped toward the scene.

"There was clearly a lot of distress," she said.

McClean said she returned to her hotel one street away after leaving the shop.

An injured person is carried in Barcelona on Thursday.

Two arrests, one deadly explosion

Two suspects who were arrested were taken into custody hundreds of kilometers apart.

One suspect was arrested in Alcanar, around 200 kilometers (120 miles) southwest of Barcelona.

Also in Alcanar, one person was killed in an explosion at a house, Trapero said, adding that incident was connected to the Barcelona attack. But Trapero didn't say whether the arrest and explosion were tied to each other.

He did say the victim is Spanish and was not on police radar.

The other Barcelona suspect was arrested in Ripoll, about 110 kilometers (68 miles) north of Barcelona and 300 kilometers (186 miles) from Alcanar.

Government officials respond

The Catalan regional government said it was holding an emergency meeting to discuss the incident.

To facilitate police operations, the Catalan emergency services urged people via Twitter to avoid going out or undertaking any other type of movement that was not "strictly necessary."

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The Spanish royal family tweeted: "They are assassins, simply criminals who are not going to terrorize us. All of Spain is Barcelona. Las Ramblas will return to be everyone's."

The Union of Islamic Communities of Catalonia expressed "condemnation and repulse" for the Barcelona attack.

"Faced with this criminal fact, the union of Islamic communities in Catalonia reiterates its full commitment to the fight against any type of terrorism, and it is expected that those responsible for these attacks may be detained and brought before the courts as early as possible," its statement said.

Barcelona officials ordered all public events to be canceled, and metro and train stations in the area were closed.

Police officers are seen near the scene of the attack.

NATO chief: We stand united

World leaders were quick to voice their condemnation of the attack and offer support to Barcelona via Twitter.

"My thoughts are with all those affected. We stand united in the fight against terrorism," said NATO Secretary General Jens Stoltenberg.

US President Donald Trump said: "The United States condemns the terror attack in Barcelona, Spain, and will do whatever is necessary to help. Be tough & strong, we love you!"

The United States condemns the terror attack in Barcelona, Spain, and will do whatever is necessary to help. Be tough & strong, we love you! — Donald J. Trump (@realDonaldTrump) August 17, 2017

London Mayor Sadiq Khan also gave his support, saying: "London stands with Barcelona against the evil of terrorism."

London stands with Barcelona against the evil of terrorism. — Sadiq Khan (@SadiqKhan) August 17, 2017

German Chancellor Angela Merkel called the attack in Barcelona "revolting," her spokesman tweeted. "We are mourning the victims of this disgusting attack in Barcelona -- in solidarity and friendship side by side with the Spanish."

European Commission President Jean-Claude Juncker branded the Barcelona attack "cowardly," adding: "We will never be cowed by such barbarism."

Las Ramblas is a predominantly pedestrianized street popular among tourists in Barcelona. Extending for about three-quarters of a mile through the center of the city, the tree-lined street is especially crowded in the summer, the height of tourist season.

The promenade passes by kiosks, flower sellers, pavement cafes and bars. It includes a number of the city's most popular sites.