Hundreds of demonstrators sat on the road as they protested at Bank junction in front of the Bank of England.
Some carried signs that said they had been “arrested for wanting a future for our children”.
Other activists from the HS2 Rebellion, a group protesting against the environmental damage that the high-speed rail project HS2 is causing, climbed a seven-storey City of London building as part of the continuing campaign.
Two men locked themselves on to the outside wall, with a banner reading: “Marsh Insurance Ditch HS2".
“Essentially, we’re targeting the profiteers, the moneymakers, the facilitators behind these eco-cidal infrastructural projects because without insurers protecting subcontractors, they wouldn’t be able to work on this," a spokesman for HS2 Rebellion said.
“It’s all of our money that is disappearing into the pockets of people in the City who we don’t usually see or hear from."
The building housed an insurance company that the protesters said had ties to the HS2 project.
The City of London Police tweeted that they had joined the Met Police and London fire officers after the protesters climbed the building on Lower Thames Street.
“A number of officers are on scene working to keep disruption in the area to a minimum," the police said. "We will keep you updated with any further developments.”
At the Bank of England, where hundreds had gathered, Extinction Rebellion co-founder Gail Bradbrook said: “This is a plea to those with power at the Bank of England to take the actions that are needed to sustain life.”
Extinction Rebellion wants the government to end all funding of fossil fuel projects and believes it and the bank are not working fast enough.
The group has carried out almost two weeks of protests as part of its Impossible Rebellion campaign.
Others sites in the City, London’s financial centre, and ministerial buildings were hit by protests, as was Buckingham Palace.
Last year, similar protests blocked Oxford Street, one of the capital’s most important shopping centres, for days.