"I'm really sorry that we have to inconvenience people, but unfortunately the government has left us no choice, " protester Andrew George said.
"In the United Kingdom, we will peacefully shut down all roads into Westminster in Central London and non-violently disrupt the government until our leaders agree to TAKE EMERGENCY ACTION NOW", the group says on its website.
They are the latest in a string of demonstrations against climate change.
Today's action comes ahead of a large direct action protest by Extinction Rebellion in London on Monday.
I'm here to force the Government to take action on climate change.
"If this equipment continues to be held, such action may well be seen as a violation of both the police's duty of care, and of the public right to protest-especially for our more vulnerable members", Extinction Rebellion said.
Extinction Rebellion members usually sit or lie down in front of traffic until they are arrested and taken away by police officers.
"Based on projections from April's Rebellion, total numbers for this Rebellion could attract between 20,000 and 30,000 people".
Police mounted on four horses have created a stand-off, blocking the path of the demonstrators, who are chanting "our water, our streets". "That's why I joined", said one demonstrator, who gave his name as Christiaan.
Police blocked the five avenues that converge on the roundabout to stop cars and buses reaching the demonstration, as it would have resulted in traffic chaos during rush hour.
Climate activists with the Extinction Rebellion movement have blocked a central junction in Berlin at the beginning of what is billed as a wide-ranging series of protests for new climate policies.
By 6:00 a.m. the police gave the first warning for the protesters to leave Stadhouderskade and go to the designated protest-area on the Museumplein, on the other side of the Rijksmuseum.
The protest was just one of a number of actions happening across the country and around the world Monday.
While traffic is closed, with activists setting up tents and playing street hockey on the bridge deck in heavy rain, bike lanes and sidewalks will remain open for the duration of the protest.
In New York's financial district, protesters spattered the Wall Street Bull and themselves with fake blood and lay prone around the sculpture to evoke their fears of a deadly environmental catastrophe.
Activists barricaded themselves to vehicles in Westminster early on Monday as the demonstrations got under way.
One group locked themselves inside a auto parked in the middle of Whitehall close to Downing Street and the Cenotaph - others chained themselves by the arms and head to a hearse parked in Trafalgar Square and glued themselves to scaffolding poles. Others are sitting down on the ground. The municipality warned for traffic delays and advised people to take other roads.
By early afternoon, police said they had carried out 135 arrests.
Hundreds of German protesters are staging a sit-in around the Brandenburg Gate.
Two weeks ago, demonstrators marched in Hungary, on the Chain Bridge across River Danube holding banners saying: "Climate protection for Hungary".