London, Twenty-one people were arrested on Monday in the UK at the start of two weeks of protests by environmental campaigners, the media reported on Monday.
Extinction Rebellion activists will demonstrate in cities around the world, including Berlin, New York and Sydney, reports the BBC.
Activists barricaded themselves inside cars in Westminster early on Monday as the demonstrations got under way.
They also locked themselves to a mock trident missile in front of the Ministry of Defence on Embankment.
A spokesperson for the Metropolitan Police said there were 21 arrests in connection with the protests by 8 a.m.
Organisers have planned to shut down key sites in central London, in addition to protesting outside government departments.
Extinction Rebellion has said that protests in the UK capital will be five times bigger than similar events in April. The organisers were expecting up to 30,000 people to participate.
The protests were calling for urgent action on global climate and wildlife emergencies.
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.