Four suspects arrested over the weekend in Sweden's second-largest city are being questioned and remain under suspicion of preparing a terror attack, prosecutors said Monday.

Authorities are being tight-lipped about the arrests in the west coast city of Goteborg, which came on the eve of the 9/11 anniversary, and were followed by the evacuation of about 400 people from an arts centre.

Swedish news agency TT said three of the suspects are Swedish citizens, two of whom were born in Africa and one in the Middle East. The fourth suspect is an African-born man with a residence permit in Sweden, TT said.

The Swedish Prosecution Authority said in a statement that investigation leader Agnetha Hilding Qvarnstrom must decide by Tuesday whether to release the suspects, or seek court approval to keep them in custody.

Prosecutors and police won't say whether the suspects are suspected of links to Islamic terror groups or other extremist networks. The prosecutors' statement said they were arrested without incident and noted that "the investigation is still in an early stage."

Ulf Ahlstedt, who has been appointed as defense lawyer for one of the suspects, said his client denies the allegations, but declined to give any other details.

"What I can say is that he's a man and he is suspected of preparation to commit a terrorist offense, which he denies," Ahlstedt told The Associated Press.

Swedish tabloid Expressen, citing an unnamed police source, said investigators suspect the men belong to a terror network with links to al-Qaeda, and that they had acquired, or were trying to acquire, firearms, explosives and hand grenades.

Police declined to comment on the newspaper's information.