British police have arrested 209 people after climate change activists blocked some of London's most famous roads in an attempt to force the Government to do more to tackle climate change.

Key points: Protesters are demanding the Government declare climate change an ecological emergency

Protesters are demanding the Government declare climate change an ecological emergency Organisers held a semi-nude demonstration in Parliament last month

Organisers held a semi-nude demonstration in Parliament last month Five arrests were made after protesters targeted a Shell building and smashed a revolving door

Thousands of environmental activists brought parts of central London, including Oxford Circus, Marble Arch and Waterloo Bridge, to a standstill on Monday.

Some stayed overnight for a second day of protest on Tuesday.

Metropolitan Police said the protests had closed 55 bus routes, affecting some 500,000 people as a result.

"Officers are out on the ground engaging with protestors and local communities to ensure proportionate policing plans are in place," police said in a statement.

"There is a need to ensure the right balance is struck between allowing the right to peaceful protest, while disruption to communities is kept to a minimum."

Extinction Rebellion, which generated headlines with a semi-nude protest in the House of Commons earlier this month, is leading the protests.

A demonstrator knits while sitting in the road as part of the group's non-violent civil disobedience campaign. ( AP: Kirsty Wigglesworth )

The group is demanding the government declare a climate and ecological emergency and reduce greenhouse gas emissions to net zero by 2025.

"I realised that signing petitions and writing letters was not going to be enough. Real action is needed," said Diana McCann, 66, a retired wine trader from south London, holding a banner in the middle of a traffic-free road.

"It's like a world war. We have to go on to a war footing."

Extinction Rebellion activists were arrested after allegedly spraying a building owned by oil giant Shell with graffiti. ( Reuters: Henry Nicholls )

Extinction Rebellion wrote to Prime Minister Theresa May on Monday outlining their demands and asking for face-to-face talks, warning that they will escalate their disruptive actions over the coming weeks unless the government acts.

"Make no mistake, people are already dying," the letter states.

"This crisis is only going to get worse … Prime Minister, you cannot ignore this crisis any longer. We must act now."

Demonstrators are calling for a full-scale rebellion to demand decisive action from governments on climate change and ecological collapse. ( AP: Kirsty Wigglesworth )

Police said five of those arrested were detained after the Shell building near the River Thames was targeted.

Two protesters scaled up scaffolding to write "Shell Knows!" in red paint on the front of the building and three protesters glued their hands to the revolving doors at the entrance.

Activists said they smashed the glass of a revolving door and caused more than 6,000 pounds ($11,000) worth of damage.

Extinction Rebellion said "over 100 brave rebels" had been arrested in total.

It said protests would continue, with demonstrations in 33 countries around the world scheduled over the coming days.

Activists have been camping out overnight in the centre of London. ( Reuters: Hannah McKay )

Reuters/AP