The Prime Minister says he wants investigations to "get the full truth out" about "horrific" allegations, that police tried to smear the family of Stephen Lawrence.
An undercover police officer claims he was ordered to find information to discredit the family, Duwayne Brooks and campaigners.
He said it happened in 1993, just months after Stephen was murdered at a bus-stop in Eltham in an unprovoked racist attack.
Both the Home Secretary Theresa May and Mayor Boris Johnson spoke about possible criminal prosecutions of Met officers, if the claims prove to be true.
We have this report from our Senior Correspondent Ronke Phillips.
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The 20-year-old was found fatally wounded on Roth Walk, near Finsbury Park, north London at around 3am on Saturday, Scotland Yard said.
Police have arrested a man wanted in connection with a burglary at the home of pensioner Richard Osborn-Brooks.
The rally comes after it emerged that the Home Office destroyed thousands of landing cards documenting the arrival of windrush-era migrants