MI6 chief Scarlett...the anti-Vietnam War rebel of Grosvenor Square 40 years ago

The British spy chief who backed Tony Blair in the Iraq war was among the protesters who fought a bloody battle with police in the famous Grosvenor Square demonstration against the Vietnam war 40 years ago.

Sir John Scarlett, now head of MI6 and a pillar of the establishment, defended violence by students outside the US embassy in London, claiming they were provoked by "vicious" police who had "clubbed" them for no reason.

The anti-Vietnam demonstration by 8,000 people turned into a bloody clash when the crowd refused to back off and were charged by mounted police.

Sir John, then an Oxford University undergraduate, wrote: "When the mounted police began charging quite viciously into the crowd 20 yards in front of us tempers were naturally lost and violence provoked as there seemed no good reason for these actions. I must protest at the behaviour of the mounted police as the park was being cleared.

"I twice saw policemen charge quite strongly at very few demonstrators who were doing absolutely nothing and both times people were heavily clubbed over the head while one of my friends saw a girl being viciously clubbed for no reason at all."

His rebellious stance is in stark contrast to his loyal support for Mr Blair in the Iraq war.

Sir John, as head of Downing Street's Joint Intelligence Committee, was accused of allowing the Government to exaggerate evidence of Saddam's weapons of mass destruction and played a vital role in Mr Blair's all-out support for the US.

During a mass protest in London on the eve of the Iraq war, Sir John was inside No10 with Mr Blair.

In the Vietnam crisis, he was on the streets with the protesters.

Despite his rebellious antics, he was not a Labour supporter. His letter to the Times, published on March 20, 1968, said: 'I was at the Vietnam demonstration but I am a Conservative while not agreeing with the American position in the war.

"It is no good dismissing the students as rowdy extremists. Many were just young people who wanted to protest against a war which they regard as wicked and it is their right to protest.

"These people had no wish to be violent. They did nothing but shout, although the turf began to fly when things began to warm and they, as I myself, had no idea of what was happening at the front. There is no reason for me to be biased about this but I feel it is necessary to say that the police became unnecessarily violent as I saw it myself."

The Grosvenor Square demonstration turned ugly after many of the 10,000 people who took part in a protest at Trafalgar Square marched to the US embassy.

Protesters broke through police ranks on to the embassy lawn, tearing up the fence and a hedge.

Stones, earth and smoke bombs were thrown.

Earlier, actress Vanessa Redgrave was allowed to enter the embassy to deliver a protest.

The protest entered anti-war folklore and inspired Mick Jagger to write Street Fighting Man.

Harold Wilson, Labour Prime Minister at the time, supported the American action, but rejected US pleas to send British troops.