Thirty activists who broke into a Swedish nuclear powerplant earlier this week to highlight lax security will be questioned by a magistrate on Friday, a judicial official said.

Most of the detainees are foreigners and public radio station SR identified some of them as Germans.

"The hearing should last all day. Thirty people will be heard from 9:00 am" (0700 GMT), court spokeswoman Birgitta Nilsson told AFP on Thursday.

The magistrate is to decide whether to press charges for illegally entering the Forsmark nuclear power plant in eastern Sweden, which could see them fined. Four interpreters are to assist the court.

More than 60 activists in total including two from environmental group Greenpeace on Tuesday entered two of Sweden's three nuclear plants to highlight security weaknesses and concerns over the safety of the decades-old facilities.

"Our two activists left by themselves to be on the evening TV news. They were never found" while on the premises, Greenpeace Sweden spokesman Daniel Bengtsson said on Wednesday.

"It's really surprising that they were able to stay hidden for so long. We thought we'd stay for 24 hours, and it's been 38 hours."

While the two were able to avoid detection at the Forsmark facility, other activists who had entered the southern Ringhals plant were found by police on Wednesday afternoon.

Eva Hallden, director of the Ringhals plant, told the TT news agency that the security response would have been more intense if the intruders had been considered dangerous.

The government has called on Vattenfall, which operates both plants, to explain the fiasco.

Environment Minister Lena Ek said during a press conference that the government would consider measures to improve security.

Some 35 percent of electricity in Sweden is generated from nuclear power.

Results of the European Union's stress tests of European nuclear reactors have shown that immediate safety upgrades costing billions of euros (dollars) are needed in power plants "nearly everywhere" in Europe.

Many of the EU's 132 reactors failed to meet international safety standards, according to the report released last week, which was commissioned in the aftermath of Japan's Fukushima disaster in March last year.