Four Greenpeace campaigners have scaled a crane beside the Houses of Parliament in Westminster to protest at the government's plans to update Trident.
Police reportedly tried to stop the activists as they began their climb
Once in position the activists unfurled a 50ft banner suggesting PM Tony Blair "loved" weapons of mass destruction.
The two men and two women plan to stay there until the Commons votes on the issue on Wednesday, said a spokesman.
They intend to phone MPs asking them to reject plans to renew the UK's Trident nuclear weapons system.
The crane, estimated to be about 200ft high, is fixed on a barge on the River Thames and is being used to replace cast-iron fascias on Westminster Bridge.
A spokesman for Interserve, which is carrying out the work for Transport for London, said the protest was a "real pain".
Police were called to the scene at 1830 GMT on Monday and are currently trying to talk to the demonstrators.
A police spokeswoman said one protester had been taken to hospital with a leg injury.
According to Greenpeace, the injured man was a cameraman travelling on the boat that dropped the climbers onto the barge.
"As the boat left the barge and came up the Thames he hurt his leg as it hit a wave," said a spokesman for the organisation.
MPs will debate and vote on Wednesday evening on the £20bn plan to replace the submarine-based Trident nuclear weapons system.