What made an accountant dedicate his life to documenting the plight of the homeless?

It wasn't the encouraging words of a teacher or friends that led Lee Jeffries to photograph the faces you see on these pages; it was a reprimand.

For more than three years, Jeffries has been taking pictures of homeless people around the world. But they're not snatched shots of the dispossessed, taken from afar: they're intimate portraits captured in uncomfortable detail, every grain of dirt, every scar laid bare.

In 2008, Jeffries visited London from his native Bolton to run the Marathon. Already a keen sports photographer, he walked around the city the day before the race and noticed a young homeless girl in a shop doorway in Leicester Square, unnoticed by the throngs of tourists. Standing a little way off, he began taking pictures of her.

"She kicked up a right fuss. She started shouting, 'You can't do that!' and 'Give me some money,'" says Jeffries, who is an accountant by profession. "I was across the street when she started shouting at me and I was embarrassed."

It was a pivotal moment for the amateur photographer: walk away, pretend it never happened; or go over to the girl, apologise, and begin talking to her.

"So I sat down," he continues. "She was in a sleeping bag and we talked. She was 18 and she'd run away from home. There were the usual problems – drugs and stuff. She had all the scars. And that's where it all started."

Since then, his project has taken him to skid rows in Los Angeles and the grimier parts of cities in Italy and France. And it has not been without its dangers: he's had a gun put to his head, been verbally abused and had large sums of money demanded for his subjects' time (little do they know at the time that he pays them whether they ask for it or not).

Like a fashion photographer who meticulously selects his models, Jeffries chooses his subjects carefully. "I don't shoot every homeless person I see. I have to see something in their eyes. Unless you feel you can get some emotion from your subject, the image just won't work. I've taken hundreds of shots of homeless people and, to be honest, lots don't work."

His words are evocative of a famous quote from the veteran war photographer Don McCullin, who also spent the best part of two decades taking pictures of homeless people: "Photography isn't looking, it's feeling. If you can't feel what you're looking at, then you're never going to get others to feel anything when they look at your pictures." As with McCullin's portraits, Jeffries' subjects are mainly captured in black-and-white and shot without artificial lighting. But he admits that, to make the end result more "artistic", a great deal of post-processing goes into his pictures.

Apart from attempting to capture the essence of his subjects, he says he thrives on the human contact he has with them: "I want to be up close, taking portraits and not telling the story of the environment they're in, but telling the story of them. I've spent days with some people and still not been able to pick up the camera because they don't allow it. You have to spend the time with them. Tell them what you're doing; tell them what it's for and six, seven times out of 10, I get a photograph. The other times I just leave."

Jeffries is honest about the reasons behind his work and the effect it's had on his view of the homeless: "I don't generally get involved in any of the politics. I make contact with these people, hopefully brighten up their day and give them a bit of money. I've got more of an appreciation for the life they have now."

Tomorrow is World Homeless Day ( worldhomelessday.org). For more from Jeffries' project, see http://ind.pn/p2U7NL