Hundreds of thousands of peace marchers flooded the heart of Barcelona on Saturday shouting "I'm not afraid" - a public rejection of violence following extremist attacks that killed 15 people, Spain's deadliest in more than a decade.

The tip-off about the imam was made informally between two police officials from Belgium and Catalonia who knew each other, a source in Catalonia's government said.

Spanish media reported that the suspects were trying to carry out a vehicle attack similar to the one that took place in Barcelona.