A series of climate change protests in London last month cost the Met Police an extra £7.5m, it has been claimed.
Nearly 1,200 people were arrested during 11 days of protests by the Extinction Rebellion group.
Met Commissioner Cressida Dick said overtime payments and the cost of bringing officers in from other forces contributed to a rise in the expense.
She also called on ministers to consider changes to “quite ancient” public order laws.
Protesters brought parts of London to a standstill in April, with roadblocks at Waterloo Bridge, Oxford Circus and Marble Arch.
Ms Dick told the London Assembly’s police and crime committee that 30 police officers were still working full time to bring cases to the CPS and through the courts.
The extra cost of policing the protests – including overtime, kit, a cherry picker and support from other forces – was at least £7.5m.
A special grant application will be made to the Home Office to recoup the funds from the government.