Most were submitted by migrant workers. The government has received more than 15,000 complaints from workers who were not paid by their employers, and more than 3,300 workers said they were forced to work long hours.

Many workers also complained that companies refused to pay social security insurance for them, which is required by law.

Figures show the number of disputes involving more than 30 workers in Shenzhen increased by 12 per cent to 512 in the first six months of the year. The number of labour disputes has been increasing for the past three years, local media reported yesterday.

Every day, the reception office is filled with bitter migrant workers looking for help.

'I thought I could earn some money here and go back home to build a new house,' said Chen Ming , a migrant worker from Hubei who worked on building a hotel.

'We have to work day and night and don't even have enough time for sleep,' he sad.

'The contractor just gave us some money for food. I haven't had my salary for eight months.

'My wife is asking me to send some money back and I don't know what to do.'

Some workers said they were asked to work long extra hours. In some cases, they have to work 120 hours extra a month, three times higher than the legal limit.

Shenzhen has been a popular destination for millions of migrant workers thanks to its booming manufacturing industry.

But a poor working environment and harsh conditions have triggered many workers' protests.

The Shenzhen government said the increasing number of petitions had affected social stability and caused disturbances to public order.

The Labour Social Security Bureau said it would assign more people to help solve the disputes.

It also warned that it would put companies which received complaints from more than 30 workers on a monitoring list and send inspectors to these firms regularly.