One arrested amid 'total chaos' of Las Ramblas Barcelona attack
A manhunt is ongoing, Spanish police added.
At least 13 people are confirmed to have been killed when a driver deliberately drove a van into pedestrians on one of Spain's busiest streets on Thursday, Spanish media reported, in what police described as a "terrorist attack".
"We were coming back from the beach and saw a scene of total chaos," Dutch holidaymaker Patrick Tuenter told The Guardian.
"People were screaming, there were a lot of wounded people. We saw a white van stopped in the middle of the street - the part where pedestrians walk down. It had driven right down the middle and thrown people onto the cobbled part of the street, down the sides."
The area around the incident was cordoned off, as police enacted what they have called "Operation Cage" to close down the area.
"Huge collision on Las Ramblas in Barcelona by an individual driving a van, many injuries," an initial statement from Catalan police said.
Police in Barcelona, Spain's second-largest city, told crowds fleeing the scene by megaphone that they were dealing with a "terrorist attack".
US President Donald Trump was quick to condemn the attack, immediately referring to the event as a "terror attack" in a tweet that will no doubt draw comparisons with his much-criticised delayed response to a vehicular attack targeting anti-fascist protesters in Charlottesville on Saturday. One woman was killed in that attack and several others injured.
The famous Las Ramblas boulevard is one of Barcelona's busiest streets, normally thronged with tourists and street performers until well into the night.
More than a thousand people have already used Facebook's "safety check" tool to offer help to those caught up in the attack, with some offering food and accommodation.
Spain has so far been spared the kind of extremist violence that has occurred in nearby France, Belgium and Germany.
'Deadliest jihadist attack'
But it was hit by what is still Europe's deadliest jihadist attack in March 2004, when bombs exploded on commuter trains in Madrid, killing 191 people in an attack claimed by al-Qaeda-inspired militants.
Vehicles have been used in several "terror attacks" in Europe in recent years, including a militant massacre that claimed 86 lives in the French Riviera city of Nice.
That onslaught in July last year and other similar attacks were claimed by the Islamic State group.
In July 2015, a hooded attacker opened fire outside a hotel in downtown Barcelona near Las Ramblas, leaving two people injured, police said.
One person was struck by bullets while the other was hurt as they tried to flee the shooter, who fled the scene. No suspected motive for the attack was given.
Spain has emerged as a potential target for jihadists, with extremist websites mentioning it for historical reasons, since much of its territory was once under Muslim rule.
Generally, authorities in Spain - the world's third largest tourism destination - remain discreet on the terror threat.
But they publicise every arrest of alleged militants, most of them detained for propaganda, recruitment for extremist groups or "glorifying terrorism".
According to the interior ministry, more than 180 "jihadist terrorists" have been arrested since June 2015 when Spain raised the terror alert level to four out of a maximum of five, in domestic and foreign operations.