London's police chief has called on ministers to consider a change in the law, after climate change protests that cost her force an extra £7.5 million.
The Extinction Rebellion group brought parts of London to a standstill in April, with roadblocks at Waterloo Bridge, Oxford Circus and Marble Arch.
Metropolitan Police Commissioner Cressida Dick said that nearly 1,200 people were arrested - 89 more than once - with more than 70 of them charged, after 11 days of protests in the capital.
She 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.
Ms Dick said the extra cost of policing the protests, including overtime, kit, cherry picker, and bringing officers in from other forces, was at least #7.5 million.
Deputy mayor for policing and crime in London, Sophie Linden, said a special grant application will be made to the Home Office to recoup the funds from the Government.
Ms Dick promised the Met would act faster to tackle similar protests in the future, but said "we are working with quite ancient laws".
She said powers under the Public Order Act are "limited" and "very slow and cumbersome" to enact.
"Serious disruption is serious disruption. Nobody can think it is a good thing that anybody who fancies doing it could just stop business going on for a long time and then be subject to non-serious offence," she said.
"These were so-called peaceful, but in the case of Extinction Rebellion, certainly in the vast majority of cases, non-violent.
"They were unlawful, but the offences that they may have been charged with are summary only and not serious and therefore the deterrent is not strong.
"So that's another thing that needs to be looked at, as to whether there should be a stronger deterrent."
She said it will be a "matter for Parliament and the judiciary", adding that it is a "very difficult balancing act".
"I don't think this sort of disruption should be not serious," she added.
Ms Dick defended her force's response to the protests, but criticised officers who were photographed skateboarding and dancing with campaigners.
"This was not professional, they took it too far in the engagement and they have been advised," she said.