Police in London said they had arrested 21 climate activists by 8am (local time) on Monday as the Extinction Rebellion protest group launched two weeks of peaceful civil disobedience to call for urgent government action to curb carbon emissions.

Reuters reporters in London's government district of Westminster saw activists block a bridge and several roads as the protests gathered pace.

Protesters were taken into custody in Amsterdam as well, where hundreds of climate change activists chanting “rebellion!” blocked traffic in downtown, defying a ban by police who began detaining them one-by-one to clear the road.

Police had approved plans by activist group Extinction Rebellion to demonstrate on condition they would not block traffic.

Scores of police using megaphones instructed the crowd to clear the road and those who did not leave were picked up and put on buses.

"Police we love you, we do this for your children," some chanted. "We are peaceful, what are you?"

"The aim is de-escalation and to break up the blockade and allow for the safe flow of traffic," police said in a statement.

"Some demonstrators will be arrested. Police also seized materials that facilitated the demonstration."

Demonstrators gathered on a main traffic artery through the city in front of the national Rijksmuseum with banners that read: "stop talking, start acting" and "tell the truth".