With his trench coat collar turned up, flat cap pulled low and a dusting of salt and pepper stubble, Paul Stephens resembles nothing so much as an archetypal veteran detective, which is what he is. He served in the Metropolitan Police for more than 30 years before retiring in February 2018 after 13 years as a detective sergeant.

Today he is an Extinction Rebellion (XR) climate activist. The protestors ridiculed as “hemp-smelling” and as the “importunate nose-ringed” in a recent speech by Boris Johnson have among their ranks lawyers, doctors and ex-police officers. As their police liaison, Stephens, who is 55, has been helping with everything from informing protestors of their rights and the consequences of illegal activity, to negotiating with river police over where a hot pink boat with “CLIMATE EMERGENCY” displayed on the side could moor on the Thames at Westminster.

We met in a café in St James Park, London, where he drank milky tea. This is where he would join other activists before protesting with them during XR’s recent two-week “Autumn Uprising” in and around Whitehall.

Stephens recalled first joining the cause after he stumbled upon a protest in Oxford Street in April, when he was “off to buy a pair of trousers”, he smiled.

“I didn’t know about it until I walked through it.” That day he listened to a couple of speakers and was impressed, and then he visited the group’s Drummond Street offices near Euston train station. “It was like a sixth form common room,” he told me. A sign on the wall read: “We welcome everyone and every part of everyone.”

Stephens was moved – “because as an ex-cop, I didn’t think I was going to be very welcome into an activists’ headquarters, and it made no difference at all. Everybody found it quite interesting, and it was still warm and welcoming. Even with hardened activists, the most important thing was that you were there and contributing.”

Yet there are activists who would rather not tell Stephens all their plans. “It looks like I’m a plant! I’m not long out of the police – people are lovely, but they don’t necessarily want to share everything, and I totally respect that.”

So far, 1,828 people have been arrested and 164 charged over the autumn campaign. At first, Stephens would accompany protestors to court. “Some of them are petrified – they’ve never been in trouble and they’re in their sixties being called a ‘defendant’.” Since June, he’s been attending protests every week as a police liaison, “liaising with people I’ve just left behind”. He recognises some of the officers on duty and says he has “a reputation now, in the police”.

A good one?

“Er, with most, I think. The trouble is, the police have an assumption of authority, so when they say something, they expect that XR people – who are predominantly white middle-class, which is a shame and we’re trying to reach out – are going to do what they’re told. I have to challenge that.”

Stephens believes that “very low-level illegality, civil disobedience, on a massive scale” is “the perfect way to protest”. He has been worrying for at least 15 years that “the human race is fucked, because we’re greedy and want more stuff, which creates more waste”. Even as a child growing up in the Essex new town of Harlow – which “was like a left-wing experiment according to my parents” – Stephens had the sense that “we’re on a finite planet”.

Yet XR strikes him as a turning point. “We’re used to looking after ourselves, our tribe, and for us to stop doing that and to treat the globe as a whole is an evolution, don’t you think?

“This is such a big threat that it will have to unite everybody, of all faiths, all politics, to have a chance, which could be a really beautiful thing.”

Then he laughed, and added apologetically, “It’s highly unlikely.”

Stephens is doing his bit, in any event. He eats meat only once a week, has given up his car and cancelled two holidays to visit friends in California and Vancouver. He would also risk arrest for the cause, “if it achieved something”. Yet his ideal protest would be alongside other ex-police officers in the movement. Climate change, he said, is “the biggest safeguarding threat to the public, and that’s why we’re here. We’ve spent all our lives safeguarding the public and all of us agree this is the biggest threat. And we’ve done police operations on a lot less evidence and information than we’ve got about climate change. The evidence is overwhelming: we have to act.”

Stephens joined the Met at the age of 19; before that, he wanted to be a fighter pilot, “back in the day when combustion engines weren’t bad!” he laughed. Although the RAF backed his pilot’s licence, hay fever put paid to that ambition. So he ended up training at Hendon Police College in 1983, the same year as current Met commissioner Cressida Dick.

After a couple of years in uniform, he took time out to live in Japan, where he studied the martial art of aikido, before working in Islington and Camden murder squads. Surveillance operations followed, then “that job got canned by [former Met head] Bernard Hogan-Howe and Theresa May’s austerity cuts”.

The rise of knife crime and county lines drug gangs were predicted when cuts to resources began back then, he told me. “The job was getting so ridiculous, so tight” that he retired, having just missed controversial pension reforms. Cuts to police services still frustrate him, especially when resources are poured into controlling peaceful protests with “intimidation” and “no communication, no negotiation”.

Geese, pigeons and a bold squirrel surrounded him as he posed for photos in the middle of the park. “The police management have a choice,” he shrugged. “They can crack down on us, or ease off.”

He continued: “To hear honesty from the hierarchy would have been really good. I’m sure Cressida in her own mind thinks she can spend her money much better on proper stuff, rather than policing Extinction Rebellion.”

Other ex-police officers who have joined Extinction Rebellion feature in “Behind the Blue Line”, a film by Rubber Republic and Extinction Rebellion: