The concert was cancelled after Spanish police alerted the Netherlands of an unspecified threat. Two men have so far been detained. Authorities say there is no link between Wednesday's incident and the recent deadly attack in Barcelona.
Police in the Dutch port city of Rotterdam detained the driver of a van with a number of gas tanks inside after a concert by an American rock band was cancelled on Wednesday night following a threat, the city's mayor said.
A second suspect was arrested by Dutch police early Thursday in the south of the Netherlands in relation to the Rotterdam threat, the Dutch public broadcaster said.
Although Spanish police tipped the Dutch to potential danger at the venue known as Maassilo, a judicial source in Spain said there was no link to the attacks in Catalonia last week.
The source said the tip resulted from an investigation by the Spanish Civil Guard that had been under way for some time.
Cities across Europe have been on high alert after Daesh claimed attacks in Spain’s northeastern region of Catalonia last week which left 15 people dead and around 100 injured.
Rotterdam Mayor Ahmed Aboutaleb said at a press conference that the van's driver, who is Spanish, was being questioned by police.
Aboutaleb warned against "swift conclusions" while noting the white van had Spanish licence plates and had been flagged for circling near the concert venue.
"It would be wrong at this moment to pile up these facts and conclude ... there was a plan to attack with gas bottles," he said.
Spanish news agency Europa Press, quoting Spanish anti-terrorist force sources, reported that the Spaniard arrested in Rotterdam had, in principle, no link to terrorism.
The gas canisters in his vehicle were apparently for domestic use, it said.
Military explosives experts were deployed to Rotterdam to examine the van, police said.
Dutch police questioned on Thursday the Spanish man detained with gas canisters in a van.
The man is being held in Rotterdam.
"The investigation is ongoing and we are questioning the man," police spokesperson Gijs van Nimwegen said. "We are working to determine his identity."
The van contained "a couple" of gas canisters and the man's driving had been "conspicuous" he said, declining to comment on Spanish police reports he had been drinking alcohol.
The National Coordinator for Security and Counterterrorism's office said the threat level in the Netherlands was unchanged at "substantial", where it has been for several years.
Rotterdam police said they took the information about a threat "seriously enough that after discussion with organisers it was decided to cancel the event."
Concert organiser Rotown said earlier on Twitter that the concert venue, a former grain silo called Maassilo, was being evacuated because of the unspecified threat.
The show hadn't yet started when the decision to cancel it was made.
Op last van de politie zijn wij genoodzaakt het concert van Allah-Las in de Maassilo vanavond te annuleren.— Rotown (@rotownrotterdam) August 23, 2017
Dutch television showed officers in body armour outside Maassilo and what appeared to be members of the band, Allah-Las leaving the venue in a white van with a police escort.
Netherlands spared so far
The Netherlands has so far been spared from the slew of terror attacks which have rocked its closest European neighbours in past years.
But amid a number of scares here in recent months, and reports that people linked to some of the attacks may have crossed briefly into the country, concerned top Dutch security and intelligence officials have been keeping a wary eye on events.
A spokesman for the Dutch justice ministry said the threat level in the Netherlands remained at "four" out of a possible five.
"Nothing has changed for the moment," Lodewijk Hekking said, meaning the threat levels remains that there is the real possibility of an attack in the Netherlands.