Two protesters have scaled a crane in Parliament Square in an apparent protest over the EU Treaty.
Protesters managed to unfurl a banner on the crane
They climbed up at about 0220 GMT and unfurled banners stating "Referendum Now" and "Give Us Our Vote".
It comes two days before MPs are set to vote on a Conservative amendment to the EU Treaty Bill, calling for a referendum on the issue.
Last week, campaigners staged a rooftop protest at Parliament against the expansion of Heathrow airport.
The Metropolitan Police was called on Monday morning to Middlesex Guildhall in the square after reports of a group of demonstrators attempting to climb the crane.
"On arrival police discovered two male protesters on the crane barge unfurling a banner," said a Met spokesperson.
"Officers are currently speaking to the protesters."
The incident follows a report suggesting that most Britons believe there should be a national referendum on the EU document, known as the Lisbon Treaty.
A mini-ballot organised by the campaign group I Want a Referendum saw more than 150,000 people in 10 constituencies vote on the issue. Some 88% were in favour of a national referendum.
A spokesman for I Want a Referendum said it was not responsible for the Parliament Square protest.