Three water cannon bought by Scotland Yard in a controversial £200,000 deal have arrived in the UK.
London mayor Boris Johnson and Metropolitan Police Commissioner Sir Bernard Hogan-Howe said the weapons were being converted for use on London's streets.
Johnson and Hogan-Howe were being questioned by London Assembly members at City Hall.
Assembly members asked whether they would be on standby during the Notting Hill Carnival but the Commissioner said the water cannon would not be ready for some months.
Boris Johnson said the devices, which were bought second hand from Germany, were at "an undisclosed location".
Scotland Yard and City Hall are still waiting for permission from the Home Secretary before water cannon can be used in mainland Britain. Johnson said there was no plan to change the way demonstrations in London are policed: