“Police detained several people suspected of involvement in a bomb alert in Gothenburg,” Gothenburg police said in a statement.

Suspects were being questioned on suspicion of preparing a “terrorist crime,” police said, but were unable to give any details of any concrete threat.

“The threat against central Gothenburg has not been warded off with today’s detentions. We stand by what we said initially, which is that a bomb is to be detonated somewhere in central Gothenburg on Saturday,” said police spokesperson Björn Blixter.

Police revealed Friday that they had been tipped off “by a credible source” that a bomb attack was planned in central Gothenburg.

“We received the right information which led to today’s detentions,” Blixter said.

The police were unwilling to confirm the identity or number of those detained.

“In view of the classification of the crime – conspiring to commit terrorist attacks – then we have to be restrictive with the information that we divulge,” he said.

Security patrols were stepped up in the city but nothing had been found by early Saturday. The police on Saturday morning deemed the threat to be credible but vague and have formed a special task force to work on the case.

Sweden’s Security Service (Säpo) said it was working closely with Gothenburg police but the terrorist alert level remained unchanged.

The terrorism alert level was raised by one point in October from “weak” (two on a scale of five) to three because of the perceived increased threat of terrorism in Europe.