Their eyes lined with black pencil and lips painted red, women in sequined saris line the labyrinth of squalid lanes that make up Sonagachi, one of Asia's largest red light districts in the old quarters of this bustling city.

In front of open sewers, they chat on mobile phones and flirt with customers, who follow them into the dark doorways of decrepit brothels, up winding staircases into tiny rooms with just a bed, television and posters of Hindu gods on the walls.

In the global battle against HIV/AIDS, sex workers like those in Sonagachi are a crucial link in a chain of infection that some 20,000 experts gathered in Washington are debating how to break -- but without having foreign sex workers there.

U.S. travel restrictions on visas for sex workers mean thousands of them have been unable to attend the annual International AIDS Conference (IAC), the world's largest forum to discuss policy on fighting the deadly virus.

In protest, sex workers from around the world have been staging a parallel conference in Kolkata -- a five-day 'Sex Worker Freedom Festival' to demand an end to the discrimination many face due to their profession.

"Sex workers are key to all policy decisions on AIDS," says Samarajit Jana from the Durbar Mahila Samanwaya Committee (DMSC), an Indian collective of 65,000 sex workers, and one of the co-organisers of the Kolkata conference.

"It has been proved that if you can succeed in controlling transmission amongst sex workers, you can be rest assured that you will not face an epidemic. They must be part of the discussion."

A study published by the Lancet Journal of Infectious Diseases in March showed that female sex workers' risk of HIV infection is 14 times higher than those of other women, adding this was a 'disproportionately high' burden of the disease.

SEX WORK IS ALSO WORK

Over the last five days, almost 1,000 sex workers from India and 42 other nations including Kenya, Mexico, Uganda, China and Indonesia been discussing access to drugs, promoting safe sex, and loopholes in HIV/AIDS policies, as well as interacting with participants at the Washington conference by video link.

But the festival's main aim is to use the U.S. travel ban to highlight the wider issue of the discrimination sex workers face and are demanding the decriminalisation of the trade.

"I chose this work. It's like any other job, but still I have no rights because society judges me and prevents me from having recognition," says 36-year-old Sapna Gayan, one of 12,000 sex workers in Sonagachi.

"Police have arrested me, clients have hit me when I ask them to wear a condom. Sex workers have no freedom to protest the abuses they face, to move and work freely. We cannot even go to big meetings where decisions about us are being made."

The United States is hosting the annual IAC for the first time in 20 years, after President Barack Obama lifted a travel ban on HIV-positive people in 2009. But immigration policy still means that those with a history of drug use or prostitution in the last 10 years are ineligible for visas.

U.S. officials, however, say this is not a blanket ban.

"Each visa decision depends on case-specific circumstances, and decisions on eligibility and waivers are only determined at the time of the interview," said an email response from the U.S. State Department.

But activists disagree. Few sex workers are given waivers, and even then, it is for a very short period of stay, they say.

On Tuesday, more than 5,000 sex workers - including heavily made-up transgenders in wigs and saris and bare-chested African men - walked through Kolkata's street, carrying banners and chanting 'U.S. government shame on you' and 'Sex work is work'.

"Sex workers' rights are human rights and the U.S. and the rest of the world need to see that," said John, a 30-year-old sex worker from Nairobi. "They can't sit in Washington and speak for us. We are part of the solution."