NICE, France—The Barcelona city center was in lockdown late Thursday afternoon after a large white van plowed into pedestrians in the historic Las Ramblas district, killing at least 13 people and injuring dozens more, with the death toll expected to rise according to authorities.

The attacker, believed to be armed, was reported to be holed up in a local bar and possibly holding hostages but there was no confirmation from the police. Authorities confirmed one suspect had been arrested about two hours after the attack but, again, further details were scant. Some Spanish media identified an alleged perpetrators, but The Daily Beast is not publishing the name until there is official confirmation.

A graphic video posted on Facebook showed a number of victims lying either dead or unconscious along the wide, tree-lined boulevard that was stained with blood. Other victims managed to sit up and were being tended to by medics or passersby.

The attack took place in the stately, elegant Las Ramblas section of the city during the height of the summer tourist season in Spain and had eerie parallels to last year’s truck attack in Nice, France, when 86 people died and hundreds were injured. Since then, we have seen vehicle attacks in Berlin, London, Stockholm, Israel, and in the United States in Charlottesville, Virginia.

Eyewitnesses reported that the van jumped the sidewalk in a pedestrian zone in Las Ramblas and careened from side to side as it barreled into tourists and local resident.

U.S. Secretary of State Rex Tillerson said the incident in Barcelona “has the hallmarks it appears of yet another terrorist attack,” and said U.S. diplomats are “currently assisting Americans in Barcelona who are affected.”

“Terrorists around the world should know the United States and our allies are resolved to find you and bring you to justice,” he added, in remarks to the press in Washington, D.C., Thursday.

Spain was the target of the worst jihadi terrorist attack in European history in March 2004, when multiple bombs went off at Madrid's main commuter train station, killing 191 people. After a long lull, as The Daily Beast reported last year, tensions have risen dramatically, and there was a widespread feeling in Barcelona—and other Spanish cities—that despite heavy police patrols something like this might happen.

Amateur video showed paramedics tending to wounded people on the street while others were taken on stretchers into waiting ambulances.

Barcelona police took to Twitter to ask social media users not to tweet out graphic pictures of the dead and wounded. Guns drawn, police could be seen on Spanish television patrolling the area looking for the suspects.

A Spanish passport belonging to someone with an Arab surname was found in what is believed to be the white van used in the attack, according to Barcelona’s Ara.cat news website.

Reports from Barcelona media indicated the attack occurred around 5:25 p.m. local time when the van plowed into people on the Ramblas’ main promenade. The driver of the van, who had what was described as a “long gun” and wearing a blue and white striped shirt, escaped. Many people took refuge in the shops along the boulevard.

In the immediate aftermath, some of the injured were covered with parasols from a coffee shop, one eyewitness said.

Restaurant worker Antonio Vito told Spanish media that he was speaking to two customers when he saw a large white van running at top speed “between 80 and 100 kilometers per hour” on the central Paseo de la Rambla. Vito said there was a stampede of people trying to escape the area after the attack.

Stanna Matias-Capet, another eyewitness, said the driver of the van wore sunglasses, had a beard and brandished a Kalashnikov.

A girl named Alba told a Spanish reporter that police came into the restaurant where she was eating and lowered the blinds so “nobody can lift them. They are looking for (the suspects.” Alba said two German girls sought refuge in the restaurant and could not “stop crying” out of fear over the attack.

Mayor Ada Colau, who was away on vacation, has returned to the city where many events for Thursday night were cancelled.

—with additional reporting by Kimberly Dozier in Washington, D.C., and Christopher Dickey in Paris