"Officers were on scene almost immediately," the force said on Twitter.
Police in London on Monday warned activists from Extinction Rebellion they faced arrest after the environmental pressure group blockaded Tower Bridge, one of the capital's most recognisable landmarks, to traffic.
The Metropolitan Police said demonstrators, who are halfway through two weeks of promised protests in the British capital, had used a van and a caravan to obstruct the world famous crossing.
"Officers were on scene almost immediately," the force said on Twitter.
"We are working with @CityPolice to get traffic moving again," it added, referring to the neighbouring City of London police department responsible for the heart of the financial district.
"We'll be warning those protesting that this obstruction of roads is unreasonable and they must disperse. If they do not move, they may be arrested," the Met said in a later update.
Extinction Rebellion kicked off a new round of disruptive protests last Monday, with thousands of climate change activists since thronging central London to demonstrate against perceived global inaction on the issue.
"Another Towering achievement from #ExtinctionRebellion as the world's most famous bridge is BLOCKED," the group's UK arm tweeted Monday.
On Friday, hundreds protested in the City of London financial hub against the funding of fossil fuel industries by lenders in the so-called Square Mile, as part of the ongoing action dubbed the Impossible Rebellion.
Police have arrested dozens over the past week, including 34 during rallies on Sunday.
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