BARCELONA, Spain — A white van mowed down dozens of people eating ice cream, window-shopping and enjoying the summer sunshine on one of Barcelona’s busiest streets Thursday afternoon, the latest vehicle turned weapon terrorist attack in Europe but the first in Spain.

The attack killed 14 people and wounded more than 100, at least 15 seriously, the Interior Ministry confirmed. Authorities believed the van’s driver was still at large and said the death toll would likely rise.

Islamic State claimed responsibility for the attack via the group’s propaganda site, known as Amaq, saying “the perpetrators of the Barcelona attack are soldiers of Islamic State and carried out the operation in response to calls for targeting coalition states,” referring to the U.S.-backed mission in Iraq. That claim of responsibility could not be verified.

The attack was the latest in a string of terrorist strikes across Europe — a low-cost, low-tech method of mass murder that presents a grave challenge for counterterrorism specialists. Some of the attacks appear to be directed by Islamic State operatives, while others are “lone wolf” operations by self-radicalized perpetrators, but officials and terrorism analysts fear they may be on the increase as the group’s territorial base in Syria and Iraq continues to shrink and its foreign operatives return to their homes in the West.

“It was clearly a terror attack, intended to kill as many people as possible,” Josep Lluis Trapero, senior police official, told a news conference late Thursday as a national manhunt continued to find the unidentified driver of the van.

Later, according to Spanish public TV, five terrorists were fatally shot by police early Friday in Cambrils, a seaside resort town about 60 miles from Barcelona.

RTVE reported that the suspects planned a similar mass-casualty attack and may have had suicide explosive belts. The terrorists injured seven people, two of them seriously, the station reported.

But many critical details of the plot remained murky Thursday night, even as news photos and cellphone videos captured a scene of carnage, confusion and despair after the van’s 50-mph rampage down one of the great public spaces in Spain. August is the height of the tourist season for the Mediterranean port city, popular in particular with British and American travelers.

Spanish police said they fatally shot a suspect just outside Barcelona after he attempted to run down two officers at a control point near the city’s exit. However, police did not confirm that the man was linked to the attack.

Several hours later, authorities reported two arrests, one a Spanish national from Melilla, a Spanish-run Mediterranean seafront enclave in North Africa, and the other a Moroccan. Police declined to confirm the identity of the man arrested.

Spanish public broadcaster RTVE and other news outlets named one of the detained as Driss Oukabir, a French citizen of Moroccan origin, The Associated Press reported. RTVE said Mr. Oukabir went to police in Ripoll to report that his identity documents had been stolen. Various Spanish media said the IDs with his name were found in the attack van and that he claimed his brother might have stolen them.

But Mr. Trapero said the van’s driver had not been identified or captured late Thursday. The arrests took place in the northern Catalan town of Ripoll and in Alcanar, the site of a gas explosion at a house on Wednesday night. Police said they were investigating a suspected link to the van attack.

A king’s consolation

Spanish King Felipe VI vowed that the country would unite in the face of the attack. Barcelona is part of the state of Catalonia, which intends to hold a referendum on independence this fall.

“Today, we are all Barcelona,” he said.

Spanish Prime Minister Mariano Rajoy called the killings a “savage terrorist attack” and said Spaniards “are not just united in mourning, but especially in the firm determination to beat those who want to rob us of our values and our way of life.”

President Trump condemned the attack on Twitter, and Vice President Mike Pence, traveling in Panama, said the Barcelona attack “shows us again that radical Islamic terrorism is one of the greatest threats that we face in the world.”

“ISIS has taken credit for this barbaric attack,” said Mr. Pence, using an acronym for the terrorist group. “But whoever is responsible should know that the United States of America, together with our allies, will find and punish those responsible, and drive the evil of radical Islamic terror from the face of the earth.”

Secretary of State Rex W. Tillerson likewise said the U.S. stands ready to assist Spanish authorities if necessary and that the incident appears to have the hallmarks of another terrorist attack.

“We offer our condolences to the loss of life and the injuries that have occurred to so many innocent people yet again,” Mr. Tillerson said.

It was the country’s deadliest attack since 2004, when al Qaeda-inspired bombers killed 192 people in coordinated assaults on Madrid’s commuter trains. In the years since, Spanish authorities have arrested nearly 200 jihadis, but the only deadly attacks were bombings claimed by the Basque separatist group ETA that killed five people over the past decade.

Although Spain had largely avoided the large-scale terrorist attacks that have hit other Western European nations in recent years, a high-level Spanish police official said authorities have been anticipating a major strike for some time.

“We have dismantled dozens of plots and conducted about 300 arrests and detentions of terror suspects over the last two years, but some were bound to get through,” the official said.

Spanish authorities have been pressing the European Union to increase cooperation with North African nations trying to pre-empt terrorist cells there from trying to infiltrate operatives into Spain and other nations along Europe’s southern border.

Scenes of panic

Thursday’s attack began when a van jumped the curb to Las Ramblas, the famed pedestrian thoroughfare, from Plaza Cataluna at around 5 p.m., and swerved back and forth for some 600 yards before it collided with a kiosk and came to a stop atop a Joan Miro mosaic, according to eyewitnesses.

Witnesses described scenes of panic with hundreds of people screaming and trying to flee the area. Others, meanwhile, hid in stores, a church, restaurants and bars on the street as police instituted a lockdown and a manhunt for the suspects.

Tom Gueller, who lives on an adjoining road, told the BBC that he fled the plaza when he saw the van mowing down pedestrians strolling down the street.

“I heard screams and a bit of a crash,” Mr. Gueller recalled, “and then I just saw the crowd parting and this van going full-pelt down the middle of the Ramblas, and I immediately knew that it was a terrorist attack or something like that.”

In the aftermath was a stream of unverified reports, including an erroneous one that said two suspects were holed up with hostages in a restaurant.

A taxi driver told TV3, the Catalan television network, that the van picked up speed after it jumped the curb.

“It was heading down making S’s in order to run over everybody it could,” said Oscar Cano.

Albert Tort, a nurse who was walking nearby, told the network that the police let him enter the scene to help before ambulances arrived.

He described a disaster.

“I was trying to give first aid to an Italian man, but he died,” he said, holding back tears. “As I walked down, I saw dead bodies on the ground, people screaming and crying.”

Since July 14, 2016, when a terrorist killed 86 in Nice, France, there have been numerous high-profile terrorist attacks involving vehicles across Europe — including in Britain and Germany last year.

One Barcelona resident expressed shock that Spain was attacked. She said it would change her life.

“I live near the Sagrada Familia” — Barcelona’s famed cathedral — “and this makes me very uncomfortable about walking near there now,” said Gemma Mondon, 51, who owns a real estate company, referring to another tourist draw. “We’re so used to it being Paris, London or Brussels, but not here.”

In the U.S., the Department of Homeland Security said acting Secretary Elaine Duke has been briefed on the situation and that department authorities have reached out to Spanish counterparts to provide support.

“We will not let terrorism become the new normal,” the department’s statement said. “Instead, such acts of violence only harden our resolve to fight back against violent extremists, bring them to justice and dismantle their networks.”

The State Department warned U.S. citizens in Barcelona to avoid the Las Ramblas area for the time being.

“Review your personal security plans, remain aware of your surroundings, especially or at large events, and monitor local news sources for updates. Be vigilant and take appropriate steps to enhance your personal security,” read a message from the U.S. Consulate in Barcelona.

• Martin Arostegui in Spain and Dave Sherfinski and Dave Boyer in Washington contributed to this article, which is based in part on wire service reports.

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