Environmental activists from the group Extinction Rebellion brought London to a standstill on Monday by erecting a large pink table in the middle of the West End.
Hundreds of protesters carrying chairs are staging a sit-in around the huge table which has the words “Come to the Table” written on it.
The stunt began right after midday and police were forced to close off several roads in the area.
Earlier, hundreds of people marched through St James’s Park, near Buckingham Palace, towards Trafalgar Square, where a marching band blocked the road.
The group converged on Trafalgar Square on Monday morning to begin two weeks of what they described as “impossible rebellion".
Officers tried to bring Monday's protest to an end by putting in place a cordon which allowed people to leave the area but not return to it.
However, this resulted in the area being occupied almost entirely by protesters, forcing police to move some of their cordons.
The Metropolitan Police had previously promised to take a harsher approach after previous Extinction Rebellion demonstrations paralysed the capital.
The protests come before the UK hosts the Cop26 environmental summit in November and after a series of UN reports warning of the increasing danger of climate change.
Gail Bradbrook, co-founder of the activist group, opened the “Impossible Rebellion” campaign with a speech expressing solidarity with nations affected disproportionately by climate change.
The two weeks of activism will take place at locations around London.
Extinction Rebellion said thousands of people are expected to take part in the protests, which aim to “target the root cause of the climate and ecological crisis".
Last year, similar rallies led to the closure of Oxford Street, one of the capital’s main shopping streets.
On Sunday, the activists climbed the outside of the headquarters of the City of London's government as they launched a series protests focused on the capital's financial district.
About 200 Extinction Rebellion protesters demonstrated in front of the medieval Guildhall, home of the City of London Corporation, which governs the city's financial centre.
Among the crowd at Guildhall was Game Of Thrones actor Jerome Flynn, who said it was “more urgent than ever” for people to take a stand together.
“I’m a bit lost for words at the moment. I’m feeling very moved to be here,” he said. “It feels needed and more poignant and more urgent than ever to come together.”
Three activists climbed the facade of the ornate Guildhall, parts of which date back to the 15th century, lit red flares and displayed a banner that said “co-liberation-freedom together".
Climate activist Greta Thunberg last week accused the UK of “greenwashing” its true environmental footprint by not including vital data in its statistics.
The Unicef index, published on Friday, found that around one billion children in 33 low-emission countries, mostly in Africa, faced a “deadly combination” of extreme weather and existing issues such as poverty, leaving them highly vulnerable.
A UN climate panel whose members include the world's top atmospheric scientists gave warning that global warming was dangerously close to spiralling out of control, with deadly heatwaves, hurricanes and extreme weather events likely to worsen in the future.