Boris Johnson has urged police to “use the full force of the law” to deal with climate change protesters.
Extinction Rebellion is currently blocking streets around London and has taken over Westminster and Lambeth bridges in the capital, bringing traffic to a standstill.
Celebrity Ruby Wax is one of several celebrities taking part in the protests at Trafalgar Square, along with actress Juliet Stevenson and models Daisy Lowe and Arizona Muse.
Around 135 people have already been arrested in the first few hours of the planned 10-day “shut down”.
The Prime Minister’s official spokesman said: “People have the right to protest peacefully but they must do so within the law.
“It is essential that people can continue to go about their business getting to and from work, visiting families and have access to vital public services including emergency services, hospitals and schools.
“The right to peaceful protest does not extend to unlawful activity – the Government expects the police to take a firm stance against protesters who significantly disrupt the lives of others and to use the full force of the law.”
But Truly Madly Deeply star, Juliet Stevenson said: “It’s a very wonderful action today.
“We can’t any longer allow governments to do this so we have to make it clear that there is no more time.
“There’s a long tradition in this country of people saying governments are not acting, we have to make them realise how urgent this is.
“I’m optimistic about the energy there is amongst people to act but I’m not hugely optimistic about government stepping up to the plate.
“They’re [the government] talking about 2050 and scientists have said we have 12 years before we’re in a place where the climate is irreversibly damaging our planet and we won’t be able to repair or fix it. We need to make them realise that time is not on our side at the moment.”
Extinction Rebellion is trying to pressure the government into action in an ongoing series of protests in major cities around the world.
The activists have been spotted doing yoga and building a “house” on the bridges, and playing cricket on blocked streets as they settle in around Westminster.
Police were out in great numbers in London around Whitehall as the planned peaceful civil disobedience that will see activists call for urgent government action to curb carbon emissions kicked off this morning.
Extinction Rebellion has predicted crowds of tens of thousands will join in its “international rebellion” in the capital in coming weeks.
Police were earlier pictured confiscated a giant sculpture of a head that was being used to block Lambeth Bridge.
Officers managed to break into an Extinction Rebellion van and arrest the driver on the bridge earlier this morning.
An onlooker told Press Association protesters had blocked the bridge with two vans and were trying to build a stage.
A small group of activists had also locked themselves to a mock nuclear missile outside the Ministry of Defence, calling on the government to redirect the funds spent on Britain’s Trident nuclear submarines towards climate policy.