By the BBC's Duncan Hewitt In Shanghai

Several hundred workers have staged fresh protests in China's north-eastern city of Liaoyang.

The workers, many laid off or retired, are reported to have gathered outside the city government's offices to call for the release of four labour leaders arrested in March.

The four had led protests by thousands of workers demanding months of unpaid wages, pensions and unemployment benefits - and an end to corruption.

They have been charged with illegal assembly and are expected to go on trial soon.

Military police were used to stop the March protests

The March demonstrations saw workers in Liaoyang and another north-eastern city, Daqing, stage what are thought to have been China's biggest protests in a decade.

They subsided after the cities were put under virtual martial law, and key workers' leaders were arrested.

Yet the workers' grievances clearly remain unresolved.

Relatives of the detained activists said between 300 and 500 people have now staged three days of protests to demand their release.

And posters placed in workers' dormitories around the town have again called for investigations into allegations of corruption by factory officials.

No negotiations

One eyewitness said police tried to remove banners from the protesters but did not make any further arrests.

Yet there is no sign of official willingness to negotiate with the workers. The four activists have been charged with staging illegal demonstrations, charges which could bring long jail terms.

And police are alleged to have beaten one man and detained another, after they tried to go to the Chinese capital Beijing last month to protest against the behaviour of the local authorities.

The sensitivity of the issue was further highlighted last week when a Chinese academic based at Harvard University was detained after visiting Liaoyang to investigate the workers' grievances.

The city lies in China's heavy industrial heartland, which has seen mass redundancies in recent years.

The governor of surrounding Liaoning province said recently that another 500,000 workers would lose their jobs in the province this year.

Fears of labour unrest have been heightened by official predictions that China's entry into the World Trade Organisation could lead to a trebling of the country's unemployment rate over the next few years.