Climate change activists blocked major routes into London on Monday, causing huge tailbacks and chaos for rush hour traffic into the British capital in the latest disruptive protest which ministers have vowed to put an end to.
About 50 campaigners from Insulate Britain, which wants the government to commit to providing insulation for 29 million homes, blocked busy routes into the city including Blackwall Tunnel in east London and a bridge over the River Thames in southwest of the capital.
Police said they had made 38 arrests for disruption of the highway and conspiracy to cause public nuisance.
It is the 11th such action by the group in recent weeks, following previous protests which have seen blockades of the busy M25 London orbital motorway and roads to the Port of Dover.
The government, which has already sought court injunctions to try to prevent the action, said on Sunday it would amend existing legislation to increase penalties for obstructing a highway to an unlimited fine, six months' imprisonment, or both, in response to the action.
"Recent weeks have demonstrated that the police need stronger powers to tackle those who illegally block motorways," transport minister Grant Shapps said. "We will not stand by while motorists are held hostage."
Insulate Britain said its action would continue regardless.
"We're more scared of what will happen when the climate crisis causes the breakdown of law and order, than we are of injunctions and prison," an Insulate Britain spokesperson said.
"The government is focussing on us rather than what's coming down the road. They need to face up to reality."