LONDON (Reuters) - 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.

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. Roadblocks were due to continue night and day at each site and the demonstrators said the protests could last at least a week.

The protests are being led by the British climate group Extinction Rebellion and are to 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 Britain’s parliament 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.

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Activists said they smashed the glass of a revolving door and caused more than 6,000 pounds worth of damage. The organisers said five people had been arrested for criminal damage.

“EXTINCTION REBELLION”

At Oxford Circus, protesters unveiled a pink boat that says “TELL THE TRUTH” and on Waterloo Bridge demonstrators brought trees, hanging baskets and skate ramps.

The protest had a festive atmosphere, with many families in attendance, and a low police presence.

Police later said that, from 1755 GMT for 24 hours, protestors would only be allowed to gather at Marble Arch to prevent “ongoing serious disruption”. A Reuters photographer said police had begun arresting protesters on Waterloo Bridge.

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.

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“Make no mistake, people are already dying,” the letter states. “In the majority world, indigenous communities are now on the brink of extinction. This crisis is only going to get worse ... Prime minister, you cannot ignore this crisis any longer. We must act now.”

Organisers of the protests circulated legal advice to anyone planning to attend, requesting they refrain from using drugs and alcohol, and asking them to treat the public with respect.

The disruption follows similar action last November when thousands of protesters occupied five central London bridges. Police arrested 85 people that day.

Rowan McLaughlin, 47, a teacher, said this week’s protests were more important that the huge pro and anti-Brexit protests in London recently.

“In Europe, out of Europe, it makes no difference if we have no liveable habitat,” he said. “We’re just going to get bigger and more annoying until the government speaks to us.”