More than 15 police vans accompanied the crowd outside the Royal Courts of Justice in London, where about 250 protestors blocked the Strand.
"We're here in solidarity with them and climate activists all over the world who are putting their lives on the line for climate justice", the spokesperson said.
The "Summer Uprising" campaign saw the group stage a "die out" at the National Museum of Ireland on Merrion Street, before carrying out temporary road blockades around Tara Street and Bachelor's Walk.
They've installed a bright yellow boat and are now setting up a protest camp to shine a spotlight on the role of Leeds' financial sector in financing polluting investments in fracking, coal and other dirty technologies.
British Parliament declared an environmental emergency in April, but Extinction Rebellion activists said not enough has been done since. It's only going to get worse.
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.More news: Lions players get little respect in Madden NFL 20 ratings
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"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".
In Glasgow, a 25-foot purple boat was placed in the city centre causing partial closures of Glasgow's historic Trongate.
One message on the vessel said: "The future you fear is already here".
In Leeds, protesters set up tents on Victoria Bridge, one of the main transport networks over the River Aire. Enough's enough: "it's time to act now".
The movement would like to see more "meaningful measures" from the government on reducing carbon emissions to net zero, protecting biodiversity, and criminalising actions that harm the environment.
The government announced in June it would commit to reaching net zero emissions by 2050.