Araceli Herrera was just a few months old in October 1968, when army snipers gunned down scores of student protesters a few streets away from her home near the run-down neighbourhood of Tlatelolco, in Mexico City. Had she been older, she would certainly have been out there covering the action. Her dream of becoming a reportera grafica got closer when, at the age of 14, she left school to work as a receptionist for a picture agency. Yet her plan to learn on the job was repeatedly frustrated. On the rare occasions when she was allowed out of the office, she was treated as a bag-carrier. Until, that is, her boss found the team short-staffed one day. "He took me aside and explained very simply about focus and exposure. Then he put a camera in my hand and sent me off to cover for my colleague."
The next day, Herrera's pictures - of the inauguration of President Miguel de la Madrid - were splashed across the front pages of three national newspapers. With her first wages, she bought herself the Nikon camera that she still uses today. It is the only one she owns. Her photographs are regularly reproduced at home and abroad - in papers including the New York Times - but Herrera, the mother of a teenage son and the only wage-earner in her family, still has to budget for her next bottle of developing fluid.
She jokes that she learned to be a photographer in 15 minutes. "I was always fascinated by the images I saw as a young girl in Mexico City, even though much of the photography at that time was quite different from what I do now. Long before I went to work, I was always lining things up in my head, making a frame with my hands to see how it would work."
Herrera's forte is an eye for the shot that others dismiss. Perhaps it helps that she is tiny and can slip into spaces where bigger people, with bigger cameras, might not. Either way, she brings a different and informed perspective to the job. As well as presidents and intellectuals, she values the mentally ill man on the street, and the community that scavenges around one of the largest rubbish dumps in the northern hemisphere. "We must show everything," she says, "because it takes such a long time to change things. This dump has been like this for years."
Her images are not perfect, even a little blurry at times, but they are all the more memorable because she manages to imbue them with a feeling of what motivates her subjects. In one shot, a man holds up a small picture - the only part of the image that is pin-sharp - of a friend who has disappeared. It is clear what the viewer should concentrate on.
For her first exhibition in the UK, at the Oxo Tower on the South Bank in London, Herrera has selected work from the past two decades - a period of tremendous upheaval, from the earthquake in 1985, which killed 20,000 people, to the Zapatista uprising in 1994. Her portraits, including a strangely shadowy-eyed Gabriel GarcIa Marquez and an eerie, skeletal incarnation of Ofelia Medina, the original celluloid Frida Kahlo, are striking, yet it is her photographs of isolated communities that stand out. While scenes of deprived campesinos are standard fare in the Mexican press, Herrera is not content with a quick trip to the mountains or jungle. Instead, she travels for weeks at a time.
Rather than presenting a uniform picture of rural poverty, she captures moments of frivolity - a family emerging laughing from a temazcal, a sort of ancient sauna, or Purepecha women taking a break during a 300km march to the capital. And the images taken in the forests of Chiapas grab the eye. As one of the first photographers to reach the troubled southern state just after the Zapatista uprising, Herrera (working for the weekly Proceso) turned her lens on the soldiers, both federal troops and the underequipped rebels.
The show's curator, Miriam Haddu, believes that Herrera - now working independently - represents a new phase in Mexican reportage. "Until a few years ago, it was unusual to find a woman working as a news photographer at this level, especially a woman from an indigenous background. Her work is unique. She looks at people with their own eyes."
"Araceli Herrera: Mexico through the lens" is at the Gallery at Oxo, Oxo Tower Wharf, London SE1 (020 7401 3610) until 24 October. Entry is free