Seven people were arrested, one of them anti-war campaigner Brian Haw, during a demonstration outside Downing Street.
Mr Haw, who has been encamped in Parliament Square since June 2001, was filming the protest when arrested.
He accused police of using "violent and humiliating force" but Scotland Yard said no complaint was made at the time.
A police spokesman said 100 people took part in the march against the Serious Organised Crime and Police Act (Socpa), which limits protests near Parliament.
Mr Haw, from Redditch, Worcestershire, said: "I was filming the students lying down in the road when one officer stepped forward, as I was walking back, and pushed the camera with his hand. It struck my face."
He said he was "dragged" by police into a police van, who pushed "my head close to the ground with my arms handcuffed high above my back".
A Metropolitan Police spokesman said they were forced to act because the march contravened the Socpa legislation.
"They did not seek authority from the Metropolitan Police Service for the demonstration," he said.
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