In London, organisers say they expect thousands to take part in peaceful acts of civil disobedience, bringing widespread disruption to the capital.
The first human roadblock closed Waterloo Bridge to traffic in both directions on Monday morning before protesters descended onto the roads at Oxford Circus and Marble Arch.
Extinction Rebellion said: "We've tried petitions, marches, letters, reports, papers, meetings, even direct actions; and global emissions have continued to rise". Activists are targeting five central locations as they demand the Government declare a climate emergency.
Eighty-five people were arrested in London in November when thousands of protesters, including families and pensioners, occupied five bridges.More news: Microsoft Confirms Hackers Could Read Outlook, MSN and Hotmail Emails
Activists said they hope to create a "festival" of action, including people's assemblies, performances, talks, workshops and food. Extinction Rebellion have organised a nationwide week of action, they are calling for a full-scale Rebellion to demand decisive action from governments on climate change and ecological collapse.
He said: "We are here tonight to declare that we do not wish to be at war".
Yen Chit Chong, from Extinction Rebellion in London, said: "This is our last best shot at survival".
Demonstrators arrived at London's Hyde Park on Sunday, some having journeyed to the city on foot in recent weeks from various parts of the United Kingdom for what is described as an International Rebellion.More news: Bharat first look poster out: Salman Khan in never-seen-before avatar
While organisers encouraged people to set up camp in Hyde Park overnight into today, they were warned they could be breaking the law by doing so under Royal Parks legislation.
More than 80 cities in 33 countries will hold similar events in protest about environmental issues, they said.
Police advised Londoners to leave extra time for their journeys because of possible delays caused by the protest.
The Metropolitan Police have said they have "appropriate" plans in place and that officers will be used "to support the public order operation during the coming weeks".More news: Stacey Dooley breaks silence over Kevin Clifton scandal