Sienna starts a hippy riot

The time, however, is not 1968 but 2007.

The eerily accurate reconstruction of the day when anger at the Vietnam conflict spilled over on the streets of Britain was part of filming for Sienna Miller's latest movie.

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In Hippie Hippie Shake, she plays Louise Ferrier, the girlfriend of Richard Neville, the editor of notorious counterculture magazine Oz.

The film is about the Australian author's time in London in the late Sixties.

The scenes capture the moment Ferrier takes part in mass peace demonstrations. Grosvenor Square was closed to allow hundreds of extras to take part in filming of the riots.

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They recreated 17 March 1968 when thousands of protesters who had gathered in Trafalgar Square to call for peace descended on the American embassy.

Led by Tariq Ali and addressed by Vanessa Redgrave, they were inspired by American protests against President Lyndon Johnson's continuation of the war in Vietnam.

Ali and his fellow activists, he recalls in his memoirs, "dreamed of using the embassy telex to cable the US embassy in Saigon and inform them that pro- Vietcong forces had seized the premises in Grosvenor Square".

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Mounted police charges and more than 200 arrests kept the demonstrators out of the embassy - which was soon transformed into the heavily guarded and fortified building it is today.

Miller, 25, is understood to have been asked to cut down on her exercise regime in keeping with the rounded physique of Sixties "chicks".

She is joined by a largely British cast playing some of the key characters who surrounded Neville in an adaptation of his memoirs, called Hippie, Hippie, Shake: The Dreams, the Trips, the Trials, the Love-ins, the Screw ups - the Sixties.

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The film is directed by Beeban Kidron, who was behind Oranges Are Not The Only Fruit and Bridget Jones: The Edge of Reason, and is adapted from the book by her husband Lee Hall.

Cillian Murphy plays Neville and Chris O'Dowd plays Felix Dennis, a fellow publisher of Oz magazine.

Both were prosecuted in 1971 for obscenity.

Margo Stilley, who starred in Michael Winterbottom's explicit Nine Songs, plays a groupie called "the Banger Sister".

Max Minghella, son of director Anthony Minghella, also appears, while Emma Booth plays Germaine Greer, who was a friend and associate of Neville.