LONDON — Climate activists glued themselves to the top of a London commuter train, blocked a major bridge at rush hour by staging a group yoga class and occupied four major London landmarks for a third day on Wednesday as part of a global civil disobedience campaign that demands government action on climate change.

Major London road junctions and tourist sites, including Marble Arch, Oxford Circus and Waterloo Bridge, have been at a standstill since Monday, as hundreds of environmental activists have staged events, formed roadblocks and camped out in colorful tents in scenes reminiscent of the Occupy movement of the past.

Further confrontations were expected after the Metropolitan Police announced their intention to limit the protests only to the area around Marble Arch, and began working to remove protesters from their encampments at Oxford Circus and Waterloo Bridge.

“We will not leave until our concerns are addressed,” said Cressida Thomas, a student from Bristol, after the police warned her to move her tent in Oxford Circus. “That could take weeks or months. The more of us they arrest the better. It will only bring more attention to the cause.’’