Twelve workers sleeping in the loft of a dry snacks store in an industrial area of Mumbai were killed early on Monday by a fire that trapped them in small space they had rented to rest.

Most of the men who died were migrant workers, said Prabhat Rahangdale, chief fire officer of the Mumbai Fire Brigade, who said they had identified 10 of the 12 charred bodies by evening.

The fire started on the ground floor and the intense heat and smoke trapped the workers sleeping on the loft, which subsequently collapsed, he told the Thomson Reuters Foundation.

Industry studies show more than 90 percent of India's workforce - an estimated 400 million people - are in informal employment. This includes labourers on farms and construction sites, as well as in shops, hotels and restaurants.

Many work in conditions that violate Indian laws enacted decades ago to protect labour rights.

Last December, six workers locked in by their employer at a bakery in Pune, about 150 km from the financial hub Mumbai, died in a fire.

Among those who died were migrant workers from the northern Indian state of Uttar Pradesh who were due to take part in an International Migrants Day celebration on Monday held by a labour rights charity.

"They had been taking pictures for a photography contest we were holding. This fire accident just shows how migrant workers live," said Amrita Sharma of Aajeevika Bureau, a non-profit lobbying for migrant workers' rights.

"They work in overcrowded, poorly ventilated spaces. Their work and living conditions are highly precarious. There is a complete void in enforcement of labour laws."