The Met is planning to buy three second hand water cannon from German police at an estimated cost of £200,000.
Speaking this morning, Boris Johnson said: "I have concluded there is broad support amongst Londoners for the use of this measure by the police in limited circumstances."
The majority of the 25 Assembly members - including Conservatives - are opposed to water cannon.
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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