"Two Swedish men were arrested ... The prosecutor suspects them of sabotage," Sven-Erik Karlsson of the Kalmar county police told AFP.
One of the men was arrested and questioned early Wednesday after routine tests at the entrance to the plant detected traces of highly explosive material on the handle of a plastic bag he was carrying.
Karlsson told the TT news agency that the men were born in 1955 and 1962 and that one of them had a criminal record, but did not disclose their identities.
A spokesman for the OKG company that runs the plant, Anders Österberg, told AFP the two were welders who had been contracted for work on one of the plant's three reactors, which had been shut down for maintenance.
He said one of them had been working inside the plant for two weeks, and that sniffer dogs would be sent in to search the area they had worked in.
"One must of course take into account the information we have about where these people have been working ... and in cooperation with the police and bomb sniffing dogs search these areas to make sure these people have not left behind any explosives," Österberg said.
"It is a security measure," he said, adding that around 100 people had been evacuated from the area where the explosive traces were detected.
The plant's two other reactors were running as normal on Wednesday, he said.
The explosive material was believed to be TATP, which is relatively easy to make and has surfaced in a number of recent terrorism investigations, including bombings in the Middle East and the London bombings in July 2005.
It was the same type of explosive that Al-Qaeda "shoe bomber" Richard Reid tried to detonate on a Miami-bound flight in December 2001, three months after the September 11 attacks in New York and Washington.
Although the recipe for TATP is complex, its ingredients can be found in simple household goods: sulphuric acid -- found in drain cleaner -- hydrogen peroxide, and acetone, often a constituent of nail polish remover.
Swedish Environment Minister Andreas Carlgren said Wednesday's security scare was a wakeup call.
"This reminds us of how vulnerable nuclear power can be to sabotage attempts and other human failures," he told the regional daily Kvällsposten's website.
Björn Engström, who has worked at the Oskarshamn plant for the past 30 years and lives nearby, told the TT news agency he was pleased the security system had worked.
"I think it is really great that they caught them. I've been annoyed at the strict security measures and all the controls you have to go through on your way to work. Now it turns out they work, so maybe I won't be so annoyed anymore," he said.
The Oskarshamn plant, which is owned by German energy giant EON, has three boiling water reactors, in service since 1972, 1974, and 1985. The three reactors produce about 10 percent of Sweden's electricity, according to the plant.
Österberg said OKG had received no threats against the reactors, and terrorism expert Lars Nicander told TT there were no known threats against the plant and no reason to suspect an organised group was behind the suspected sabotage attempt.
"We are however taking all necessary measures to verify this ... and we are going over our security routines," Österberg said.
Nuclear power accounts for nearly half of all electricity production in Sweden, which has 10 working nuclear reactors.