London's Southwark Crown Court heard that with his arms flailing and eyes glazed, former public schoolboy James McLarty presented a "frightening" picture to police trying to control rioters.

McLarty, 27, who was wrestled to the ground outside the Square Mile's Liffe building, was jailed after admitting violent disorder.

The court heard that when the drug eventually wore off, he was shocked to learn what he had done.

Douglas Allen, defending, told the court his client, who lived in a caravan off south London's Old Kent Road, had led the life of a traveller since leaving public school at 18.

Thousands crammed into the Sqaure Mile

McLarty, he said, had joined the Reclaim The Streets demonstration expecting it would be no more than a large street party.

To begin with everything went well, but then he began to feel tired, decided he needed a "pick me up", and asked friends for some amphetamine.

"Instead he was given Ketamine, a horse tranquilliser, and much more powerful than speed. It had an awful effect on him," said Mr Allen.

"He felt weightless, started to hallucinate and eventually blacked out completely."

He said his only recollection after regaining consciousness was of running towards police.

"The image of him running towards officers must have been frightening," the barrister added.

Judge Paul Focke QC, said drug-taking or drinking could be used as a "shield to justify or excuse violence".

He added: "You were shouting and screaming and lunging at the police.