Environmental campaigners are blocking some roads across the UK in protest against "inaction" on climate change.
Extinction Rebellion are using five boats to stop traffic in Cardiff, Glasgow, Bristol, Leeds, and London.
The group, which is calling on the government to declare a climate emergency, said it was beginning a five-day "summer uprising".
Police said road closures could hamper the ability of the emergency services to respond to incidents.
It comes after Extinction Rebellion staged an 11-day protest in April that brought several parts of London to a standstill.
Some of the 1,000 people arrested during that protest appeared in court this week.
The five boats unveiled on Monday were each named after an environmental activist and bore the message "act now".
A blue boat was placed in front of the Royal Courts of Justice in central London, and protesters practised yoga and meditation in the middle of the road.
Transport for London said several buses had been placed on diversion due to the protest.
Extinction Rebellion said the protesters were there "to demand the legal system take responsibility in this crisis, and ensure the safety of future generations by making ecocide law".
"We also stand in solidarity with climate activists around the world who are sacrificing their freedom to fight for climate justice," it added.
The group is calling on the Metropolitan Police and Crown Prosecution Service to drop cases against those arrested during the April protests.
Outside Cardiff Castle in Wales, a number of Extinction Rebellion members parked a green boat in the street, causing severe delays to bus services.
Demonstrators also set up tents on grass in front of Cardiff City Hall.
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."
Protesters set up camp on Bristol Bridge with a pink boat bearing the message "tell the truth", as Avon and Somerset Police and traffic management used concrete blocks to close the road to traffic.
Chief Inspector Mark Runacres, an area commander at Avon and Somerset Police, said the force had cancelled officers' rest days to make sure it had "sufficient resources" during the protest.
"Any unplanned and lengthy road closure could impact on the ability of emergency services to respond to incidents," he said.
"We... are factoring this into our plans so we can continue to keep the public safe,"
He added officers would be "robust" in dealing with any anti-social behaviour and disorder.
Campaigners in Glasgow blocked Trongate at the intersection of Gallowgate and High Street with a 25ft purple boat.
One message on the vessel said: "The future you fear is already here".
Glasgow City Council said Trongate was closed to all eastbound traffic between Albion Street and High Street.
The local authority urged road users to consider taking other routes and said there could be congestion on surrounding streets.
Meanwhile, on Victoria Bridge in the centre of Leeds, activists unveiled a yellow boat.
One demonstrator, Alex Evans, 43, said: "My eldest child is nine years old and for each of those nine years I've watched her future get steadily worse while everyone waits for everyone else to do something on climate change.
"Now we're out of time and we can see climate breakdown all around us. Enough's enough: it's time to act now."
Extinction Rebellion says the UK must act immediately to halt biodiversity loss and reduce greenhouse gas emissions to net zero by 2025.
The government announced in June it would commit to reaching net zero emissions by 2050.