Drivers were left at a standstill after climate group Insulate Britain staged their 10th day of protests on two major motorways in London.
Around 30 activists blocked roundabouts at Junction 3 of the M4 near Heathrow Airport and Junction 1 of the M1 at Brent Cross.
The incidents left drivers in west and north London beeping out of frustration after being unable to move for sustained periods.
Eight of the demonstrators had been released from police custody on Thursday after previously blocking the M25 at Junction 30 in Essex, while others, who were arrested earlier this week, were also part of the manifestations.
The Metropolitan Police said the protesters blocked the M4 at around 8.27am. Officers were on the scene in 13 minutes and the road was cleared by 9am, with 13 people arrested for obstruction and conspiracy to commit public nuisance.
It has pledged to continue its actions until the government makes “a meaningful statement indicating that they will insulate all of Britain’s 29 million leaky homes by 2030”.
The group said in a statement: “It’s incomprehensible that the government is continuing to delay action on home insulation when we urgently need to cut our carbon emissions, eliminate fuel poverty and help hard-working families with their rising energy bills.
“Added to which industry is crying out for the government to show some leadership and get behind a national retrofitting strategy. Come on Boris: get on with the job.”
The government obtained an injunction last week which means anyone blocking the M25 could be found to be in contempt of court, which carries a maximum penalty of two years in prison or an unlimited fine.
Insulate Britain admitted that its actions this week “are in breach” of the injunction.
Sir Stephen House, Deputy Commissioner of the Metropolitan Police, told the London Assembly’s Police and Crime Committee on Thursday that he is concerned police forces are under so much pressure to quickly clear the protests that “officers are putting their lives at risk”.
He went on: “The most recent one I saw had officers running between articulated lorries that were moving on the main carriageway of the M25.
“We cannot be doing that. We cannot put people’s lives at risk. My officers’ lives at risk and indeed the demonstrators’ lives at risk.
“We have to look out for that first.
“But we have been very quick in moving these people and arresting them.”
He added: “They started off on the slip roads, which is bad enough, but we’ve now moved on to the main carriageways of the M25, which is absolute lunacy.”