At least 13 people were killed and 100 injured 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.

Citizens of 24 countries were among those killed and injured in Barcelona including one Belgian national confirmed to have died.

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.

"It was clearly a terror attack, intended to kill as many people as possible," Josep Lluis Trapero, senior police official, said.

Hours later, police said they had shot dead "four suspected terrorists" and left another injured in Cambrils, a city south of Barcelona.

The regional government of Catalonia, where both Barcelona and Cambrils are located, also confirmed the incident in the early hours of Friday, which police had earlier qualified as a "possible terrorist attack."

Here is everything we know.

What happened in Barcelona?

A white Fiat van, reportedly rented, rammed into pedestrians outside a kosher restaurant on a busy street in Barcelona shortly after 5pm on Thursday.

The vehicle van veered onto the promenade and barreled down the busy walkway in central Barcelona for 500 metres, swerving back and forth as it mowed pedestrians down and turned a picturesque tourist destination into a bloody killing zone. 

Victims were left sprawled in the street, spattered with blood or writhing in pain from broken limbs. Others fled in panic through Las Ramblas, screaming or carrying young children in their arms.

Lawyer and University of Glasgow rector Aamer Anwar was walking La Ramblas when he heard screaming.

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

Mr Anwar said: "I was 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. "

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."

source: AFP