Controversial plans to arm riot police in London with water cannon will be be debated at a public meeting at City Hall this evening.
The Met has released this video which demonstrates past situations when it says the use of water cannon would have been considered:
London mayor Boris Johnson backs the idea but says he wants to hear the views of Londoners before making a final decision.
Tonight's meeting - which will hear from senior police officers - is part of what the mayor calls a period of "public engagement".
The proposal by the Metropolitan Police Service to buy three second-hand water cannon from Germany is bitterly opposed by the London Assembly.
The assembly voted overwhelmingly on Friday assembly to reject the £200,000 plan.
Boris Johnson's official Deputy Mayor for London, Victoria Borwick, was among four Conservatives who voted with the opposition.
The Liberal Democrats and Greens are firmly against the idea. Labour accuse the mayor of rushing a decision and are calling for a formal public consultation.
Assembly members are expected to join a protest rally outside City Hall ahead of the 7pm meeting.
Other speakers will include Dietrich Wagner, a German pensioner who was partially blinded by water cannon during a protest in Stuttgart.
The Home Secretary is expected to decide whether to issue a license for water cannon in London next month.