Migrant workers in Hong Kong have joined a mass protest against what they called "modern-day slavery."

Hong Kong police are investigating allegations that a 23-year-old Indonesian domestic worker had been tortured by her employers.

The case has provoked outrage over abuses suffered by the city's army of foreign domestic workers.

A group representing Indonesian migrant workers in Hong Kong said that Erwiana Sulistyaningsih came to the city in May and suffered beatings until returning last week to Indonesia, where she's being treated in hospital.

The family who employed Ms Sulistyaningsih slapped and punched her every day, according to Indonesian Migrant Workers Union vice-chairwoman Sringatin.

She said the employers only let Ms Sulistyaningsih go back to Indonesia when she could no longer work because of her injuries and gave her just 70 Hong Kong dollars (£5.46).

She had been so badly beaten that she needed a friend to help her at the airport because she couldn't walk properly, said Ms Sringatin.

Photos show scabs and lacerations to Ms Sulistyaningsih's face, hands and legs and blackened skin around her feet.

Ms Sringatin said her employers permitted her to sleep only four hours a day and insisted that she cleaned in a particular order.

"If Erwiana cleaned the toilet before cleaning the bedroom, the employer would beat her," she said. "When the employer called Erwiana and she didn't hear, the employer would also assault her."

Police said they were investigating after receiving a complaint from a placement agency about abuse by an employer.

Indonesian consulate spokesman Sam Aryadi said Ms Sulistyaningsih was undergoing physical and psychological examinations in Indonesia and may return to help with the investigation.

There are about 312,000 foreign domestic workers in Hong Kong, with Indonesians accounting for about half and Filipinos making up most of the rest. The majority are young women.

A report by Amnesty International in November slammed the Hong Kong and Indonesian governments for allowing conditions that left women working as maids vulnerable to abuse and exploitation, including restrictions on movement, physical and sexual violence, lack of food and long hours.