Protesters blocked off five major bridges in central London as part of a so-called “rebellion day”.
Organisers said thousands have gathered in central London to demand the government takes greater action on climate change.
Demonstrators occupied Southwark, Blackfriars, Waterloo, Westminster and Lambeth bridges, after a week of action by campaign group Extinction Rebellion.
Southwark Bridge has since reopened, according to Transport for London.
Large groups of people holding banners began congregating on the five bridges from 10:00 GMT before blocking off the traffic.
Tiana Jacout, of Extinction Rebellion, said the blockages were “not a step we take lightly” but “if things continue as is, we face an extinction greater than the one that killed the dinosaurs”.
“We have tried marching, and lobbying, and signing petitions. Nothing has brought about the change that is needed,” she said.
According to the group, 6,000 people have joined the protests and 22 have been arrested so far.
Two female protesters on Westminster Bridge, who did not want to be named, told the Press Association they “truthfully believe we’re all heading for extinction”.
“Climate change is so important, it’s coming over so fast and nothing is being done,” they said.
It is believed about 50 people have been arrested for taking part in action over the last week.
The Met said the protests had caused “significant traffic disruption” and emergency service vehicles “are currently being hampered from getting across London.”
It said it had requested all protesters move to Westminster Bridge where the force would “facilitate a lawful protest”.
On Wednesday, protesters glued themselves to the gates of Downing Street.
At the start of the week, activists blockaded the UK’s energy department by chaining themselves together on the pavement.
Is the protest fair?
By Roger Harrabin, BBC Environment Analyst
We haven’t seen a British green group quite like this before. It thinks marching with placards has failed, so it’s aiming to make mayhem instead.
But have the protestors picked the right target?
The UK is in the leading pack of nations in cutting the CO2 emissions that are over-heating the planet.
The Climate Change Act locks Britain into reducing greenhouse gases by 80% by 2050, based on 1990 levels.
And the government has kept pace with the step-by-step targets so far, mostly by stopping coal -burning for electricity.
It promises to meet future targets too – although its advisors warn it has to improve by getting more electric cars on the road, and making homes and businesses more energy efficient.
The protestors say the targets will be breached if the government spends £30bn on new roads, encourages fracking and looks to expand aviation even further.
Climate change demands a seismic shift in society, they say. And they’re not seeing that yet.
Follow Roger on Twitter @rharrabin