More than 20 people have been arrested in London at the start of two weeks of protests by environmental campaigners.
Extinction Rebellion activists will demonstrate in cities around the world, including Berlin, New York and Sydney.
Organisers have planned to shut down key sites in central London, in addition to protesting outside government departments.
Extinction Rebellion says protests in the capital will be five times bigger than similar events in April.
The protests are calling for urgent action on global climate and wildlife emergencies.
Activists barricaded themselves to vehicles in Westminster early on Monday as the demonstrations got under way.
Police were seen cutting two protesters from a car that had blocked Victoria Embankment, while campaigners also locked themselves to a mock Trident missile outside the Ministry of Defence.
Meanwhile, crowds of campaigners blocked Westminster and Lambeth bridges.
A spokesperson for the Metropolitan Police said there were 21 arrests in connection with the protests by 8am.
Extinction Rebellion organisers are expecting up to 30,000 people to take part in the fortnight-long demonstrations in the capital, which form part of an “international rebellion”.
Similar protests in the UK earlier this year brought major disruption to London and resulted in more than 1,100 arrests.
Up to 60 other cities around the world may also be disrupted in simultaneous events from Monday, according to a spokesperson for the group.
Campaigners are expected to block sites around Westminster including government departments, roads and bridges, and hold a sit-in at London City Airport.
The Houses of Parliament and Trafalgar Square are also expected to be targeted, alongside processions and marches.
Activists will call on government departments to detail their plans to tackle the climate emergency.
Police in Australia and New Zealand have already arrested dozens of Extinction Rebellion activists on Monday.
Some 30 campaigners in Sydney were charged with committing offences after hundreds of protesters blocked a busy road.
The latest arrests in London come after the Met police arrested 11 people during the weekend.
A spokesperson for the force said eight people were arrested on Saturday after previously reporting 10. They have all been released under investigation.
One woman and two men were arrested on Sunday on suspicion of conspiracy to cause public nuisance. The men remain in custody while the woman has been released under investigation.
More than 1,000 people attended an “opening ceremony” at Marble Arch on Sunday night, which featured meditation and dancing.