At least 13 people were killed on Thursday when a driver deliberately slammed a van into crowds on Barcelona's most popular street in what police confirmed was a terror attack.

The attack, the latest in a wave of vehicle rammings across Europe in recent years, caused panic on the streets of Spain's largest city and drew condemnation from world leaders.

"We can confirm there are 13 dead and more than 50 injured," regional interior minister Joaquim Forn said in a tweet.

Catalan police said they had arrested one suspect, and denied earlier reports that the perpetrator was holed up in a bar near the scene.

Here is everything we know.

A white van, reportedly rented, rammed into pedestrians outside a kosher restaurant on a busy street in Barcelona.

Police released a photograph of a man called Maghrebi Driss Oukabir, who is suspected of renting the van which was used to crashed into pedestrians in Las Ramblas.

He said a shopkeeper told him five or six people were badly injured and described the scene as "chaos".

Mr Anwar said: "I had been to the Cathedral and walking down Las Ramblas for something to eat. Part of it was in the shade so I decided to keep walking down and literally within 10 seconds there was a crashing noise.

"I turned around and people were screaming - I could see a woman screaming with her kids - people started running and jumping into shops. I ran for about 50 or 100 metres and stopped to see what was happening. The police were very quickly on the scene and getting people to move back.

"I could see chaos right at the top area and I spoke to a shopkeeper who had run down and was screaming. He was Bengali so I spoke to him in Urdu and he said a van had driven into a crowd and he thought there were five to six people very seriously injured."

Mapped: Barcelona van attack

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

U.S. Secretary of State Rex Tillerson offered assistance to authorities in Spain and said U.S. diplomats in Spain were helping Americans there. He vowed the United States would never relent in tracking down terror suspects and holding them to account.