Pursued by shadowy security men in plain clothes, New York filmmaker Nanfu Wang mounts the staircase of a decrepit, gloomy apartment block in China, gasping for breath as she reaches the upper floors.

The director has been under constant surveillance since she returned to the country of her birth to film the human rights campaigner known as Hooligan Sparrow, who has been highlighting sexual violence against children.

"Hurry! Hurry! I'm so scared," Wang hisses as she ushers Sparrow's teenage daughter into a safe house to escape the men who have just arrested the girl's mother.

The gripping scene comes near the end of three months of jaw-dropping footage depicting China's state security apparatus at work across the country's vast southeast, which Wang shot and smuggled out in the summer of 2013.

Edited into "Hooligan Sparrow," a feature-length documentary that comes across more as "found footage" thriller than political reportage, it is expected to shock audiences when it hits US theaters in July.

Followed wherever she went, questioned by secret police, threatened with violence and intimated by organized mobs, Wang captured every detail of her time as an "enemy of the state."

She was forced to go on the run after security agents visited her family and friends demanding to know her whereabouts.

"I've never had illusions about fairness in China's justice system or the accountability of its government," the director told AFP.

"But I never expected to see ordinary people turn on their neighbors who were fighting for their rights."

The filmmaker carried a discreet backpack with a small point-and-shoot camera, a pair of glasses with an embedded micro camera and a small audio recorder, taking all of her footage with her everywhere she went.

"I always was afraid that my footage would be seized and destroyed, or that it would be seen by the authorities and used against my subjects," she said.

- 'Corruption has no limit' -

Wang -- who grew up in a remote farming village in southeastern China and moved to the United States as an adult -- decided to return when she heard of Sparrow, a divorcee and single mother whose real name is Ye Haiyan.

Now 41, Ye was in the middle of a social-media campaign highlighting a string of child-abuse scandals in Chinese schools, pushing for the trial of a head teacher who had raped several students in a hotel.

Ye posted a picture of herself on the southern island of Hainan holding a poster that read "Principal, get a room with me -- leave the school kids alone."

Her post went viral and the principal was eventually sentenced to 13 years in prison for the rape of four girls aged 11 to 14.

One victim's parents claim in the documentary that the government monitored their calls and told them not to speak to journalists or lawyers, and that plainclothed officials followed them wherever they went.

Ye was supported at the time by lawyer Wang Yu, one of China's few human rights specialists who has since been detained for nearly a year without trial.

"China is so corrupt that it has become fashionable for government officials to have sex with young girls," she tells Wang on camera. "The corruption has no limit."

Days after Ye's Hainan protest, 11 people stormed into her home in Bobai, Guangxi province, and began to beat her because she had exposed abusive conditions in local brothels.

The documentary shows Ye screaming at them and chasing them out with a meat cleaver before she is arrested and held for 13 days over accusations that she assaulted three of her attackers.

- Harassed and threatened -

A constant thorn in the side of officials bent on maintaining social order, Ye began handing out condoms and offering legal services to sex workers in 2006.

She took part in a controversial stunt in 2012, volunteering as a free-of-charge prostitute for two days to highlight the plight of China's sex workers, estimated to number up to six million.

She lives under "residential surveillance" and travel restrictions in her hometown of Wuhan in eastern China. The authorities confiscated her passport in November 2014.

Wang managed to smuggle her footage out of China by handing a flash drive to friends flying to the United States after she was followed during an earlier attempt to send it through a shipping office.

"I've lived in China most of my life and I've always been interested in issues related to sex workers' rights," she says, "so I was curious to learn more about this woman and what motivated her."

Now back in New York, the filmmaker says she began researching Ye and discovered that they shared similar upbringings in poor farming communities with limited access to education.

"I appreciated her respect for people whom Chinese society rejected, and I shared her desire to understand their lives more deeply," she says.

However, nothing prepared her for the experience of filming "Hooligan Sparrow."

"I never expected to be attacked by screaming mobs just for filming on the street," she said.

"I never expected to be interrogated by national security agents, and that my family and friends would be harassed and threatened by secret police. But this is the China I saw."