Two Greenpeace activists who broke into a Swedish nuclear plant managed to remain undetected for 38 hours before leaving the site voluntarily, the environmental group said Wednesday.

The pair were part of more than 60 activists who on Tuesday morning 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," Greenpeace Sweden spokesman Daniel Bengtsson said.

"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 eastern 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 showed 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.