"The situation is very tense," Andre Muggiati, one of the group's Amazon campaigners, said by telephone from Manaus.

"Our staff can't leave because there's no security."

Local police did not immediately return a phone call to comment on the situation.

The incident, the second time in nearly two months that Greenpeace activists have been harassed in the Amazon jungle, underscores the often violent conflicts over natural resources between farmers and loggers on one hand and peasants and Indians on the other.

The loggers in Castelo dos Sonhos, some 700 km (435 miles) southwest of Altamira in northern Para state, demand that Greenpeace leave behind a 13-metre (43-foot) tree trunk it was transporting to an exhibit on global warming in Sao Paulo and Rio de Janeiro.

Police and soldiers protected the Ibama building but would not guarantee the security of the activists outside, a Greenpeace spokeswoman said.

Troops had been patrolling the area after a US-born nun and human rights activist, Dorothy Stang, was murdered on a remote jungle track in 2005 by gunmen hired by ranchers.

Around 30 loggers used two trucks on Tuesday afternoon to block the Greenpeace convoy, forcing the environmentalists to seek refuge in the Ibama office.

"The tree, which was burned illegally, symbolizes the rapid destruction of the Amazon ... and was to draw attention to the urgent need of stopping deforestation and reducing Brazil's emission of gases causing global warming," Greenpeace said in a statement.

The federal government withdrew its authorization to transport the tree trunk early on Wednesday, Muggiati said.

"They gave in to pressure from the loggers," he said.