Eighty-five people have been arrested as thousands of demonstrators occupied five bridges in central London to voice their concern over the looming climate crisis.

Protesters, including families and pensioners, began massing on five of London’s main bridges from 10am on Saturday. An hour later, all the crossings had been blocked in one of the biggest acts of peaceful civil disobedience in the UK in decades. Some people locked themselves together, while others linked arms and sang songs.

By 2pm the blockade of Southwark Bridge had been abandoned and protesters moved from there to Blackfriars Bridge, where organisers said they were soon to move west towards Westminster Bridge.

Demonstrators occupied Southwark, Blackfriars, Waterloo, Westminster and Lambeth bridges.

The Metropolitan police said all the bridges had since reopened

and that most of the arrests had been for obstruction under the Highways Act.

Afterwards, demonstrators gathered in Parliament Square to hear speeches. Roger Hallam, one of the strategists behind the actions, told the Guardian he felt the protest had been fantastic.

“This is total prediction stuff, mass participation civil disobedience,” he said. “They can’t do anything about it unless they start shooting people, and presumably they won’t do that.”

The day was due to end with an interfaith ceremony outside Westminster Abbey.

The move is part of a campaign of mass civil disobedience organised by a new group, Extinction Rebellion, which wants to force governments to treat the threats of climate breakdown and extinction as a crisis.

“The ‘social contract’ has been broken … [and] it is therefore not only our right but our moral duty to bypass the government’s inaction and flagrant dereliction of duty and to rebel to defend life itself,” said Gail Bradbrook, one of the organisers.

Alice, 19, from Bristol was one of those blocking Westminster Bridge.

“I took the coach at 3am to make sure I didn’t miss it,” she said, “and I’m so glad that I did. It’s a tiny personal inconvenience and, having made it, I get to be part of a rebellion.

“This moment will be remembered in the history books, when we finally stopped allowing our leaders to take us over the cliff.”

Jenny Jones, the Green party peer, joined the protest on Westminster Bridge. She backed the nonviolent direct action taken by demonstrators.

“We are at the point where if we don’t start acting and acting fast we are just going to wipe out our life support system,” she said.

“It’s fine to think we are a rich country, the sixth biggest economy in the world, but actually we won’t do any better than anywhere else because climate change will massively affect us too.

“Basically, conventional politics has failed us – it’s even failed me and I’m part of the system – so people have no other choice.”

Father Martin Newell said on Blackfriars Bridge: “What brought me here is the climate emergency, the extinction emergency and my faith in God who created all this and whose creation we’re destroying and crucifying … I’m called as a Christian to protect our neighbour who’s being abused.”

In the past two weeks more than 60 people have been arrested for taking part in acts of civil disobedience organised by Extinction Rebellion ranging from gluing themselves to government buildings to blocking major roads in the capital.

However, those disruptions were eclipsed on Saturday, when organisers say 6,000 people took part in protests.

“It is not a step we take lightly,” said Tiana Jacout, one of those involved. “If things continue as is, we face an extinction greater than the one that killed the dinosaurs. I don’t know about you, but I’d rather be a worthy ancestor.”