"We are here to protest because of the danger that Finland will build more nuclear power," said Sirpa Paakkonen. "It is dangerous, shortsighted and completely unnecessary."

The peaceful demonstration, which police said was significantly larger than the 3,000 people initially expected, was believed to the country's largest since the early 1990s when Finns protested against unemployment and EU membership.

"Everyone has been nice and calm and the great weather has increased attendance," said senior police officer Pekka Hook.

It was on April 26, 1986 that a reactor at the Chernobyl nuclear complex in Ukraine exploded and caught fire, spreading a radioactive cloud across Europe in the world's worst nuclear disaster.

The protestors said they hoped the demonstration would help convince members of parliament to vote in late May against the government proposal to construct a fifth reactor.

According to a poll conducted earlier this month 94 MPs of the 200-member house support the plan while 88 are against and the rest undecided.

The five-party coalition government, which includes the Green party, says the best way to satisfy increasing energy demand while ensuring Finland meets its greenhouse gas emissions obligations under the Kyoto protocol is to build the country's first new nuclear reactor for more than two decades.

Opponents say the health and environmental risks are too great, and other energy sources should be favoured. In 1993 a similar proposal was rejected in parliament.

The European Union's Environment Commissioner Margot Wallstrom said recently that Finland could meet the emission targets without building more nuclear power by relying more on natural gas and renewable energy.

Finland has four nuclear reactors at two installations, supplying about 30 percent of total electricity needs.