AMSTERDAM (Reuters) - A bus with Spanish license plates and containing gas bottles was found near a concert hall where earlier on Wednesday a rock concert was canceled due to a threat, Rotterdam's mayor said.
Ahmed Aboutaleb told a news conference it was not clear whether the threat and the bus were connected, though Spanish police had tipped Dutch authorities to a potential threat.
The bus driver was taken into police custody for questioning, Aboutaleb said.
He warned against "swift conclusions".
"It would be wrong at this moment to pile up these facts and conclude: thus there was a plan to attack with gas bottles, et cetera, because that was the picture last week in Barcelona. I would be careful with that."
He said that police and prosecutors had taken the threat against the concert seriously and it would now take time to investigate.
A bomb squad was examining the bus.
Police said the concert, which was to feature Californian band Allah-Las, was canceled at around 7 p.m., shortly before doors were to be opened for guests, after a tip from Spanish police.
As concert-goers were sent away from the Maassilo, a former grain silo complex on the Maas river that has been converted into an event hall, the venue was cordoned off.
No arrests or detentions were reported until the bus driver was detained at around 9 p.m.
The National Coordinator for Security and Counterterrorism's office said the threat level in the Netherlands was unchanged at "substantial", where it has been since 2013.
Spanish police declined comment.
Incident not connected to Barcelona attacks: source
A judicial source told Reuters the tip given by Spanish authorities to their Dutch counterparts was the result of an investigation by the Spanish Civil Guard which had been under way for some time and had no direct relation to the two vehicle attacks in Barcelona and Cambrils in Catalonia that killed 15 people.
A separate police source said the man arrested in Rotterdam was Spanish.
(Reporting by Toby Sterling and Julien Toyer; Additional reporting by Bart Meijer and Jesus Aguado; Editing by Matthew Mpoke Bigg, Sandra Maler and Adrian Croft)