A driver deliberately smashed a van into a crowd on Barcelona's most popular street on Thursday, killing at least one person before fleeing to a nearby bar, police said.
Officers in Spain's second-largest city said the ramming on the popular Las Ramblas boulevard was a "terrorist attack" and a police source said one suspect had left the scene and was "holed up in a bar".
Regional interior minister Joaquin Forn said that at least one person had been killed and 32 others were wounded, warning however that the toll could rise.
A local government source earlier said two people had died.
Vehicles have been used in several terror attacks in Europe in recent years, including a jihadist massacre that claimed 86 lives in the French Riviera city of Nice.
The famous Las Ramblas is one of Barcelona's busiest streets, lined with shops and restaurants and normally thronged with tourists and street performers until well into the night.
Police said there had been a "huge collision" between a van and pedestrians on the thoroughfare and a police source said officers were seeking a total of two suspects.
- 'Bodies on the floor' -
Xavi Perez, who works in a sports shop near the site of the attack, described a scene of carnage.
"When it happened I ran out and saw the damage," he told AFP. "There were bodies on the floor with people crowding round them. People were crying. There were lots of foreigners."
Witness Aamer Anwar told Britain's Sky News television that he was walking down Las Ramblas, which he described as "jam-packed" with tourists.
"All of a sudden, I just sort of heard a crashing noise and the whole street just started to run, screaming. I saw a woman right next to me screaming for her kids.
"Police were very, very quickly there, police officers with guns, batons, everywhere. Then the whole street started getting pushed back."
- 'Mini stampede' -
Spain had so far been spared the kind of extremist violence that rocked nearby France, Belgium and Germany.
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 extremists.
Ethan Spibey, a charity director on holiday in the city, said he and several others had locked themselves in a nearby church after Thursday's van ramming.
"All of a sudden it was real kind of chaos... people just started running screaming," he told Sky. "There was kind of a mini stampede.
"It seems like a lot of people have taken refuge in shops and local cafes."
The United States offered its assistance to Spain and was providing consular assistance to Americans in Barcelona.
"Terrorists around the world should know -- the United States and our allies are resolved to find you and bring you to justice," Secretary of State Rex Tillerson told reporters in Washington.
- String of vehicle attacks -
The Nice onslaught in July last year and other similar attacks including the carnage in Paris 2015 were claimed by the Islamic State jihadist group.
In another deadly vehicle attack in December, 12 people were killed when a man driving a truck ploughed into a crowd at a Berlin Christmas market.
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 jihadists, 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.