"We detained a person who was of Swedish nationality," a spokesman for police in the popular Spanish Mediterranean resort city told AFP.
Barcelona's city hall said the small white truck -- which had dozens of orange gas bottles stacked on the back in plain view -- had been stolen.
There was no indication as yet whether this was an attempted attack or just a plain robbery.
"When people shouted at him, he would laugh and make offensive gestures with his hand," she told AFP.
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Police did not say whether there were any injuries.
Around 20 police officers and at least six police cars were on site, and several gas bottles were lying on the ground, an AFP correspondent reported.
The trace of at least one gunshot was visible on the windscreen of the truck.
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