For most people, volunteering tends to involve manning phone lines, shaking a charity collection bucket or perhaps doing a spot of community gardening.

But for a few who give up their time, the experience can be rather more eventful.

Take Zac Kozlowski, for whom lending a hand has involved taking down criminals, chasing car thieves and even rubbing shoulders with the Queen.

The 22-year-old, who works as a Royal Free Hospital security officer in his day job, has also learned how to respond to chemical spills and made countless arrests during his spell as a special constable with the Metropolitan Police.

“The duties are varied, there is so much going on in London and it’s such a challenge,” he said.

“People have got this perception that special constables are not utilised as much as we are.

“It’s not just walking up and down the high street.”

Mr Kozlowski is one of the capital’s 4,500 specials – voluntary ­officers with the same powers and uniform as normal police, who give up at least 16 hours a month to help fight crime.

He is able to juggle police duties with his job because the hospital is signed up to the Met’s “employer supported policing” programme.

The scheme – which celebrates its 10th anniversary this month – sees employers give staff paid time off to serve as specials.

In his police role, Mr Kozlowski is based at Kentish Town police station, in Holmes Road, and is often assigned to response teams that deal with 999 call outs.

He says he has lost count of the number of arrests he has made, but the most serious was for grievous bodily harm with intent.

“Some guy was running up and down the road attacking people with a weapon in Euston,” he said.

“He gave himself up and it was lucky he did, because he was absolutely enormous.”

If the man had resisted arrest, Mr Kozlowski could have called upon his training in “takedown manoeuvres”, one of the many skills he has picked up.

Other memorable shifts have seen him working “in close proximity” to the Queen at the State Opening of Parliament, learning how to deal with chemical ­accidents, and chasing car thieves who crashed a Vauxhall Corsa into a house and then escaped on foot. “That was a very stressful shift,” he said.

After joining the Royal Free in Pond Street, Hampstead, at 17, he signed up as a special constable two years ago, undergoing six weeks of intensive training before donning the uniform. He says the hospital gains because of the police training, which helps him interact with the public in his day job.

“I’ve gained a lot of confidence, a lot of exposure and life experience,” he added.

“You get involved in things that you would never otherwise see – the other side of what goes on out there.”