Members of Extinction Rebellion have blocked roads in London and four other cities in a new wave of demonstrations demanding faster action against climate change.
Hundreds of environmental protesters obstructed the street outside London's Royal Courts of Justice on Monday with a big blue boat bearing the words: "Act Now".
They are calling on the government to halt wildlife losses and cut greenhouse gases to net zero by 2025, as well as for charges against more than 1,000 people arrested during climate protests earlier this year to be dropped.
Boats are also being used to "disrupt central spaces" in Glasgow, Cardiff, Bristol and Leeds as the protest group begin a series of planned week-long protests in a "summer uprising".
Activists say they are staging a series of "creative acts of civil disobedience", which includes blocking bridges and roads as well as holding talks, workshops, people's assemblies and family-friendly activities.
In April, members of the climate change group blocked several London roads and bridges over 10 days of action designed to alert the public and politicians to the "climate emergency".
"This is the next phase of the Rebellion, and it will be as large, if not larger, than the International Rebellion in London, but this time distributed across the UK.
"For it to be successful, we need everyone's commitment," a statement detailing the action said.
In Glasgow, protesters used a 25ft purple boat to block Trongate, covering the intersection of Gallowgate and High Street, by the Merchant City clock tower.
Leeds' financial district was also targeted to draw attention to the links between banking and the climate and ecological emergency.
In Cardiff, Extinction Rebellion members parked a large green boat outside Cardiff Castle and held banners reading "Act Now" and "Climate Emergency", delaying commuters during the morning rush hour.
Tents were also set up on the grass in front of Cardiff City Hall - the home of the Welsh capital's local government - as campaigners are expected to camp there ahead of more disruption in the coming days.
Campaigners handed out leaflets that said they were protesting "to prevent the breakdown of humanity's life support system, the Earth", adding they were calling for the UK government to create a "national assembly" to implement climate change solutions.
Stephen Lingwood, 37, from Extinction Rebellion Cardiff, said: "People are dying right now of climate chaos in places like India. It's only going to get worse.
"We're at the beginning of the sixth mass extinction and a climate genocide and the government's inaction is, in my view, criminally irresponsible."
Britain aims to achieve net zero carbon emissions by 2050.