John McBeth – In jumper, skirt and sandals, Dita Sari looks more like a rural schoolteacher than a trade unionist. But working out of a converted house in the backstreets of east Jakarta, the 28-year-old former political prisoner and university drop-out is rapidly emerging as a key figure in Indonesia's fledgling labour movement as it struggles to emerge from three decades of stagnation and oppression.

For a woman who found her calling in Indonesia's underground and now goes about her work with missionary zeal, the big pink and white Amnesty International poster on her office wall probably says it all: "Hands Off Dita Sari." It was printed back in 1996 when Sari was beginning a six-year jail term for organizing what was then an illegal strike in the Tandes industrial district of Surabaya, Indonesia's second-biggest city.

Two years before, at the tender age of 21, Sari had formed the National Front for Indonesian Workers' Struggle in defiance of then-President Suharto's laws forbidding independent labour movements. Using the same secretive methods she learned as a pioneering member of the left-leaning People's Democratic Party, she now set about running the National Front from her cell – initially at a detention centre in the East Java city of Malang and later at the Tanggerang Women's Prison on the outskirts of Jakarta. In the end, Sari served only half of her sentence, thanks to the collapse of Suharto's New Order regime in mid-1998.

But her time behind bars steeled her resolve. Today, her union has 22,000 members (about half of whom pay dues) in 14 provinces across Java, Sumatra, Sulawesi, Kalimantan and Bali. Most are women working in factories producing textiles, shoes, food and beverages. But she's just taken maritime workers into her fold and she makes no secret of her ambition to build as big a union as possible.

Born in Medan, North Sumatra, to middle-class parents, Sari moved in 1988 to Jakarta to finish high school and studied law at the University of Indonesia.

Her political activities, starting with small campus discussion groups, and her prison sentence ensured that she never completed her degree. But she doesn't seem to have any time for regrets.

Although then-President B.J.Habibie embraced freedom of association and other International Labour Organization resolutions in 1998, it was another two years before she got around bureaucratic roadblocks and finally succeeded in registering the National Front. First it was the Manpower Ministry's refusal to allow the union to base its constitution on social democratic principles, rather than Pancasila, the state ideology. Then the bureaucrats complained about the "political words" in the manifesto. "It took three ministers before we were recognized," she says.

Sari claims to have no political ambitions and says that two years ago her union made a conscious decision to stay away from party affiliations.

But those attitudes could change if the labour movement is able to consolidate over the next five years and become a force for change. "Running a union is not going to be enough for Dita," says a Western labour analyst, who has followed her career. "I think she can probably look forward to being a leader in the political field."

In some ways, Sari says, things were simpler under Suharto – "there was no horizontal conflict then, everything was the state or the military against the people." Now she points to a confusing picture: remnants of Suharto's New Order, what she calls "false reformists," President Abdurrahman Wahid and his political manoeuvrings and a divided student movement. But, there is also the sense that she wouldn't have it any other way.