Two explosions sank the Greenpeace flagship Rainbow Warrior in Auckland harbor last night, killing one crewman and scuttling a planned antinuclear demonstration by the environmentalist group.

"There must be a very strong presumption of sabotage," said Greenpeace's British chairman, Bryn Jones, in London, as reported by Reuter news service.

The Rainbow Warrior sank in four minutes after the twin blasts.

Auckland police said divers searched the half-submerged hull of the converted trawler, which keeled over after two blasts below the waterline at Auckland's Marsden wharf.

The vessel sailed into New Zealand Sunday with 11 people on board. It was to have led a protest flotilla to the French nuclear test site at Mururoa atoll.

Police said the cause of the explosions was not immediately known. Several people were flung or jumped into the harbor as the 130-foot flagship of the international environmental group went down.

A Greenpeace spokesman said the dead crewman was a Portuguese photographer named Fernando Pereira, a veteran of several Greenpeace campaigns. The other 10 members of the crew were reported safe.

The ship, a converted trawler bought in 1978, has led a worldwide campaign against such targets as Soviet and Icelandic whaling fleets, dumpers of nuclear waste off Britain, and the French nuclear test sites in the Pacific.