Under sunny skies, activists sang songs or held signs that read "There is no Planet B" and "Extinction is forever" at some of the capital's most iconic locations.

Thousands of environmental activists paralysed parts of central London on Monday by blocking Marble Arch, Oxford Circus and Waterloo Bridge in a bid to force the government to do more to tackle climate change.

Environmental protesters from the Extinction Rebellion group gather around a pink boat as they take part in a demonstration at the junction of Oxford Street and Regent Street in London on April 15, 2019.

Roadblocks will continue night and day at each site and the demonstrators say the protests could last at least a week.

The protests are being led by the British climate group Extinction Rebellion and will involve demonstrations in 33 countries around the world over the coming days.

"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, which generated headlines with a semi-nude protest in the House of Commons earlier this month, has warned its members that some of them could be arrested for taking part in non-violent civil disobedience.

The group is demanding the government declare a climate and ecological emergency, reduce greenhouse gas emissions to net zero by 2025 and create a citizen's assembly of members of the public to lead on decisions to address climate change.

At the Shell building near the River Thames, two protesters scaled up scaffolding writing '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 ($7,900) worth of damage. At least one person was arrested by police for criminal damage.