The five detained activists - three Filipinos, an American and a Canadian - were among some 30 Greenpeace campaigners who staged the protest in the northern coastal town of Sual after a cargo vessel arrived with a new shipment of coal.

Police took into custody the three Filipinos after they scrambled on top of a crane and then rappelled down and unfurled a sign reading, "Clean Energy Now".

The American and the Canadian were detained when they tried to stop plant guards from lowering their three colleagues.

Police held them for three hours before releasing them, Greenpeace officials said.

Sual, in Pangasinan province, 190 km (188 miles) north or Manila, is the site of one of the Philippines' biggest coal-fired power stations. The plant is run by a U.S. subsidiary.

The environmentalists sailed into Sual on board the Greenpeace flagship Arctic Sunrise and circled around the pier on inflatable rafts during their protest.

A security guard had fired into the air to warn the group against getting closer.

The Arctic Sunrise is to stay in the Philippines for about two weeks before proceeding to Thailand.

The protest was part of Greenpeace's campaign to promote use of wind and solar power ahead of the Earth Summit in Johannesburg in August.