Political activist Peter Tatchell has been freed on bail after his arrest for running out in front of Tony Blair's car to protest against his stance on Iraq.
Tatchell was held by bodyguards
Mr Tatchell said the prime minister looked "shocked" after he ambushed his motorcade as it drove through London's West End.
The human rights campaigner, who had a placard reading 'Arm the Kurds, Topple Saddam', said: "I held on to the underside of the car and was eventually dragged away by about eight officers."
A police spokesman said that the prime minister's security had not been breached and confirmed that a 51-year-old man had been arrested at 1745 GMT.
Mr Blair's car was forced to stop when Mr Tatchell jumped in front of it.
The car's number plated was almost ripped off
The prime minister had been on his way to meet his German counterpart, Gerhard Schroeder, at the Royal Academy of Art.
Mr Tatchell was tackled by uniformed police and bodyguards who jumped from Mr Blair's car.
He said: "I ran in front of the Prime Minister's car, forcing it to a halt.
"The Prime Minister looked shocked and then the security people jumped out of the car and took hold of me."
Mr Tatchell, who is best known as a gay rights campaigner, said he was opposed to western military intervention in Iraq.
He said Kurdish and Shia rebels should be armed to topple Saddam Hussein's regime.
A Downing Street spokesman said: "This is a matter for the police."