It is midnight on the streets of Perth and four women are teetering arm-in-arm along the High Street, giggling and animated. Like most of the revellers, they've had a few drinks and seem in good spirits. The friends spot two men dressed in dark jackets and caps which bear the words "Street Pastors". Tonight, the pastors are accompanied by a photographer. One woman gleefully steals the pastor's cap and, laughing, puts it on as she grabs the pastor and demands to be photographed.

Only once the camera stops flashing does she stand back, read the jacket and ask the men what they are doing. Calmly, the pastors repeat the explanation that they have given dozens of times that evening: they are members of various local churches who give up Saturday nights to walk the streets of Perth, speaking to people who need someone to listen.

The atmosphere changes. The woman's face crumples and tears collect in her eyes. "So you're Christians?" she asks, her voice cracking. "Well, can you tell me why my brother killed himself on Christmas Day? You tell me how I'm ever supposed to get over that." It transpires that this is the first time she has been out since the tragedy and is, beneath her lively demeanor, overwhelmed by grief. The pastors are listening to her but her friends are keen to get to the next pub - they don't want to see their friend upset and without coats they're feeling the cold. They disappear into the night.

Michael Archibald, one of the squad of pastors that has been operating in Perth for the last two months, watches her go. "She wasn't ready to talk yet," he says. "Maybe we'll catch up with her again later. I think we can help her." He is obviously affected by the woman and, later, he ponders aloud whether they should have pursued her further.

The exchange is one of many that takes place over the evening. Usually the conservation is light, and often triggered by the flip-flops the pastors carry to give to girls whose feet ache from dress shoes. On other occasions, simply stating who they are can cause a person's veneer of either anger or joviality to crumble and stories of heartbreak and hopelessness pour out. Michael admits that they often get drawn into long, highly personal conversations with individuals. Alcohol merely oils the wheels of this transformation.

"We listen, principally, and we help and we befriend the people on the street," says Michael, a trainee minister and retired chartered accountant. "A lot of folk are very sad and they think that nobody cares for them at all. When you're out there and you help them a little bit, it makes a huge difference to most people. We're not criticising them at all. I think they just want to tell their story and for us to listen to it."

The pastors usually stop short of offering advice, although they give out mobile numbers so people can contact them again. "If people want to come back and speak to us, then that's no problem, but out on the street at two in the morning, they're not so ready to receive advice." The pastors regularly give out phone numbers for housing departments and social services.

Pastors now operate in Inverness, where they first hit the streets in January this year, and in Aberdeen, where the 26-strong team from 10 different churches began work in October 2007. The Ascension Trust in London is the parent company of the street pastors and it is in London where street pastors are most established, having operated there since 2003. The trust claim that in streets where pastors work, crime has been reduced by an average of 84%.

In Perth, the pastors meet at North Church on Mill Street in the centre of the city between 9.30pm and 10pm every Saturday. Around 16 pastors volunteer regularly and there are usually half a dozen on a typical night. They chat about the previous week and Sandy Scrimgeour, a retired deputy chief constable, calls the police to let them know that they are heading out and to ask if there have been any incidents. The pastors usually go to the busiest parts of the city but stay clear of fights and stay within the range of CCTV cameras. They hope that by just being there, their peaceful presence may help to defuse tensions, but they have a code word for when they feel a situation is about to turn nasty and they should leave. The fact that nobody can remember what the codeword is demonstrates how often they have found themselves at the epicentre of a fight. For security, they are linked to each other with walkie-talkies with a red emergency button linked to police, but again, so far this remains unused by any pastor in the UK.

Not everybody wants to chat. A group of teenagers on a bench fall silent when the pastors approach, but they don't appear to be offended. The pastors are still raising their profile and endeavour to greet everybody they meet, which leads to some bemused stares but, generally, most people respond. Michael spots a pair of young girls - who can't be any older than 12 years old - standing waiting at a bus stop. He has noticed them a few times and is worried that they might be involved in child prostitution. Tonight they are alone but previously they have been in the company of a group of older men. A few seconds later, the girls quickly walk away and disappear round a corner. Down one street, an ambulance alerts the group to an incident. Dots of blood on the pavement lead to a large, congealing pool in a doorway, next to a pub. The police are taking statements and drinkers are moving back inside. There is nothing to be done here.

To become a street pastor, individuals undergo 12 training sessions. They are taught about psychology, how to read a situation and techniques for defusing a heated one. Security tips are also given by police, including how to walk the streets without ever losing sight of each other and what to do when given drugs or weapons. They learn to stand so that their backs are always facing different directions to display the words "Street Pastors" to people approaching from different directions. Part of their work is to pick up glass and bottles from the pavement and put them into bins, which they see as clearing the pavements of potential weapons.

On the streets of Perth, the reaction is fairly positive. With a population of around 50,000, the city's relatively small size means that the pastors will come to know people. Hairdresser Lianne Bryant, 23, is on a night out with a large group of friends. "You don't come across trouble that often but there are hotspots," she says. "The taxi queue is where a lot of it happens." Lianne believes the pastors are gradually becoming known in the city. "Everybody knows about the flip-flops. As soon as they see the pastors, they start asking, Where are the flip flops?'"

Andy Munns, 50, stops to chat on his way home. "I think it's brilliant. It's definitely the way to get through to people - to get out on the streets and get a bit of banter," he says. Amanda Croll, 31, a full-time mum, lives locally and says: "When you go out in a bigger city and come back here, you realise how small and relatively safe Perth is. Saying that, you do hear of attacks. It does make me feel safer to think that they are out here, it's a good gesture. The fact that they are putting their trust in us, by coming out on the streets, is a great thing and most people will want to show that the trust is not misplaced."

There are now 19 areas across Scotland where pastors are being trained including Arbroath, Helensburgh and Dunfermline. "I think it's what the church should be doing," says Michael. "I really like that there is none of this po-faced churchy nonsense about it." Certainly, meeting people in a familiar environment seems to be opening the channels of communication in Perth. "People have never seen the church out there on the street meeting them - that comes as a bit of a surprise to them, as does the fact that we are human. I thought we'd get nasty little jibes and whistled at and name-called but none of that has happened. Instead, the conversations we get into are the deepest conversations I've ever been in - much deeper than I get in the Kirk."