A man has been arrested over the seemingly motiveless stabbing of a community policeman as he sat stuck in traffic near London's Victoria station.

The 39-year-old police community support officer (PCSO) was slashed across the arm and thumb as he tried to defend himself.

The suspect, who is in his 30s, was arrested after officers launched a massive manhunt, which also included sniffer dogs.

An eye witness told how a man walked up to the marked police van in Victoria Street and lunged at the driver through the window just before 10am yesterday.

"I saw a white man in his 20s walking down Victoria Street as if he was crossing the road," said Tracey Harmer, an assistant manager at a store in Victoria Street.

"He got to the middle and then started walking down the centre line following a police van.

"The van was stationary in traffic and he caught up with the driver's side window. He put his arm up and the next minute he looked like he was punching the driver, but it turned out he was stabbing him.

"I shouted out, 'Oh my God, the policeman's getting attacked'. We looked out and then the man was running off towards the (Westminster) cathedral with a policeman chasing him."

The officer, who was on patrol wearing a stab-proof vest at the time of the attack, has been treated and released from hospital.

The suspect is being questioned at a central London police station.

Yesterday morning after the attack at the junction of Victoria Street and Bressenden Place - a short walk from Scotland Yard's headquarters - police cordoned off a large section of the busy ring road, causing traffic delays.

Unison, the public service union, has called for all PCSOs to be given stab vests.

"This incident shows that the work of PCSOs can be dangerous," said Ben Priestley, Unison's national officer with responsibility for PCSOs.

"(It) reinforces the need for all forces in England and Wales to provide their PCSOs with protective equipment, including stab vests."