Police detained the men — three of whom are Swedish citizens — in a sting in the west coast city of Goteborg on the eve of the 10th anniversary of the Sept. 11, 2001 attacks, and a prosecutor said they were being interrogated on suspicions of preparing a terrorist act.
About 400 people were evacuated from an arts center in connection with the arrests, but police haven't said whether the building was the target of the alleged plot.
Swedish authorities didn't name the suspects, but court documents show they are aged 23 to 26 and living in the Goteborg area. Three are of Somali origin and one was born in Iraq. One is a Somali citizen and the rest are Swedish nationals.
Swedish tabloid Expressen, citing unnamed police sources, said investigators suspect the men of connections to al-Shabab, an al-Qaida-linked insurgent group in Somalia, and that they had acquired, or were trying to acquire, firearms, explosives and hand grenades.
The Swedish security police declined to comment on the newspaper's information.
Ulf Ahlstedt, who has been appointed as defense lawyer for one of the suspects, told The Associated Press that his client "is suspected of preparation to commit a terrorist offense, which he denies." He declined to give other details.
Lawyers for the other three suspects didn't return calls seeking comment.
Chief prosecutor Agnetha Hilding Qvarnstrom must decide by Tuesday whether to release the suspects, or seek court approval to keep them in custody.
The SAPO security service said the Goteborg police and a national police anti-terror unit were involved in the arrests Saturday.
Goteborg police said they had evacuated the Roda Sten arts center, located beneath the city's landmark half-mile (930-meter) Alvsborg bridge, because of a threat deemed to pose "serious danger to life, health or substantial damage of property."
The Alvsborg bridge runs over Goteborg's harbor to connect the mainland with the island of Hisingen. The six-lane passage is a vital link from the Norwegian capital Oslo to southern Sweden.
SAPO wouldn't specify the target of the alleged plot, but said after the arrests that there was no reason for public concern and maintained its terror alert at "elevated." It has remained at that level since being raised from "low" in October.
Once peaceful Scandinavia is no longer insulated from terrorism. Danish authorities have foiled a series of plots against a newspaper that printed caricatures of the Prophet Muhammad in 2005, sparking riots in Muslim countries.
In December, suicide bomber Taimour Abdulwahab blew himself up in downtown Stockholm among panicked Christmas shoppers, injuring two people and shocking the country.
In a report detailing the extent of extremist Islamist networks in Sweden, ordered months before that attack, the SAPO security agency had downplayed the risk of terror attacks in the Nordic country. Activity among radicalized Muslims in Sweden is primarily directed toward supporting militants in other countries, including Somalia, Afghanistan and Pakistan, it said.
On July 22, a self-styled anti-Muslim extremist blew up a car bomb in downtown Oslo and then massacred youth gathered for a Labor Party summer camp, killing 77 people in the region's worst violence since World War II.