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Emma Thompson joined climate change activists in central London

“Our planet is in serious trouble,” Thompson said amid a crowd of 300 activists. She addressed them from a pink boat in the middle of London's Oxford Circus. (Pic EPA)

Film star Emma Thompson joined climate change activists in a central London shopping district on Friday to read poetry praising Earth's bounties, part of five days of protests which clogged the British capital with transport snarl-ups.

Organizers Extinction Rebellion have called for non-violent civil disobedience to force the British government to reduce net greenhouse gas emissions to zero by 2025 and stop what they call a global climate crisis.

The protests did not cause major travel disruption on Friday during one of Britain's biggest holiday weekends, but police said they have now arrested more than 682 people.

Extinction Rebellion blocked several locations in central London this week after staging a semi-nude protest in parliament earlier this month.

“Our planet is in serious trouble,” Thompson told reporters amid a crowd of about 300 activists, according to eye witnesses. She addressed them from a pink boat in the middle of London's Oxford Circus as shoppers and tourists milled past.

“We are here in this island of sanity and it makes me so happy to be able to join you all and to add my voice to the young people here who have inspired a whole new movement,” said Thompson, one of Britain's most acclaimed actresses who has won two Academy awards.

She was one of several actors who read poems celebrating the beauty of nature.

The activists formed a human chain around the boat, with one attached to its main mast making it extremely difficult for the police to dislodge him.

After a painstaking operation, they removed him and secured the boat, but had difficulty transporting the vessel away because more protesters continually sat in their path.

“The serious disruption the demonstrations are causing to people in London and beyond is unacceptable and we completely understand the concern it is causing to those who are disrupted by it,” police said in a statement.

Thompson's appearance followed a demonstration near Heathrow Airport earlier, where a group of around a dozen teenagers, some as young as 13 and 14, held a banner alongside a busy road which read: “Are we the last generation?”

Some of the teenagers wept and hugged each other, although they were far outnumbered by police.