For the satisfaction of taking a sledgehammer to Tony Blair's gates, a protestor got his car blown up by police.

Only a few bits of paint came off the Prime Minister's Downing Street gates in the attack at 6.20am today. Armed police immediately arrested the 58-year-old man and confiscated his sledgehammer.

Explosives officers searched the man's luxury car, which was parked close by, and blew it up in a "controlled explosion" on the site. It is routine for officers to search and, if necessary, destroy suspicious objects, a police spokeswoman said.

As a precaution, the street from Parliament Square to the Horse Guards was also closed off and searched for other attackers or terrorist devices.

The reason for the man's feat is unclear, she added. "The incident is not being treated as an act of terrorism."

Whitehall was reopened at 7.30am.

The man is still in custody at a central London police station.

He is not the first to attack the gates, which were put up in 1989 after Irish terrorists bombed the Inglis Barracks.

In February demonstrators dressed as judges threw white paint at the gates. They protested against the limited responsibility of an inquiry into British intelligence on Iraq's alleged weapons of mass destruction. Five people were arrested for criminal damage.