Insulate Britain have apologised for the disruption caused to motorists and emergency services by blocking the nation’s highways.
Protesters from the Extinction Rebellion offshoot brought traffic to a standstill in the capital on Monday despite three court injunctions already in place.
“We cannot imagine undertaking such acts in normal circumstances. But we believe that the reality of our situation has to be faced.
“The collapse of the climate is happening around us. We face economic chaos and the breakdown of law and order in a matter of years.
“Take us to court, charge us, and put us in prison. We are more fearful of the loss of our country than we are of the courts. Throw injunctions at us, but we are going nowhere, there is nowhere to go.”
The Prime Minister called the group, who are demanding that all UK homes be fully insulated by 2030, “irresponsible crusties” on national radio Tuesday morning.
Tempers boiled over on Monday as one frustrated driver urged other motorists to “just drive over them” in clashes with a dozen activists sat on Wandsworth Bridge.
An ambulance driver was also seen pleading with the group to be let through before physically dragging one of them to the side.
One woman was filmed by LBC begging to be let through the Blackwall Tunnel blockade to see her mother who was being rushed to hospital in Canterbury.
On Tuesday morning, Mr Johnson told LBC: “There are some people who call those individuals legitimate protesters.
“They are not. I think they are irresponsible crusties who are basically trying to stop people going about their day’s work and doing considerable damage to the economy.”
His comments come ahead of Home Secretary Priti Patel’s speech to the Conservative Party Conference on Tuesday in which she laid out new measures to deal with demonstrators deemed to be disruptive.
Ms Patel, speaking at the Conservative Party Conference in Manchester, told members: “It is because of our commitment to putting the needs of the hard-working, often silent, majority first that I will not tolerate so called eco-warriors trampling over our way of life and draining police resources.
“Their actions over recent weeks have amounted to some of the most self-defeating ‘environmental’ protests this country has ever seen. Freedom to protest is a fundamental right our party will forever fight to uphold. But it must be within the law.
“Measures already going through Parliament will ensure these criminals can be brought to justice for the disruption they are causing.
“But we are going further to close down the legal loopholes exploited by these offenders.
“So, today, I can announce I will also increase the maximum penalties for disrupting a motorway, criminalise interference with key infrastructure such as roads, railways and our free press, and give the police and courts new powers to deal with the small minority of offenders intent on travelling around the country, causing disruption and misery across our communities.”