The St John Ambulance Brigade said it treated 86 people for injuries. Fifty were taken to hospital including up to 25 police officers.

The trouble followed a big rally in Trafalgar square, when an estimated 10,000 demonstrated against American action in Vietnam and British support for the United States.

The mood at the rally was described as good humoured. The violence broke out when the protesters marched to the US embassy in Grosvenor Square.

The embassy was surrounded by hundreds of police. They stood shoulder to shoulder to cordon off the part of the square closest to the embassy.

Tensions rose as the crowd refused to back off and mounted officers rode at the demonstrators.

The protesters broke through the police ranks onto the lawn of the embassy, tearing up the plastic fence and uprooting parts of a hedge.

During a protracted battle, stones, earth, firecrackers and smoke bombs were thrown.

One officer was treated for a reported serious spinal injury, another for a neck injury.

One officer had his hat knocked off and was struck continuously on the back of the head with a stick from a banner as he clung, head down, to his horse's neck.

Earlier the actress Vanessa Redgrave was allowed to enter the embassy with three supporters to deliver a protest.

She had been one of the speakers at the rally in Trafalgar Square.

Labour MP Peter Jackson, has said he will be tabling a private question for answer by the Home Secretary about what he called "police violence".

He told The Times newspaper: "I was particularly outraged by the violent use of police horses, who charged into the crowd even after they had cleared the street in front of the embassy."

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