SHANGHAI - Shanghai authorities warned residents on Friday not to conduct illegal protests against its high-speed "maglev" train project, a day before a visit to the city by British Prime Minister Gordon Brown.

Hundreds of people demonstrated against the project last Saturday in Shanghai's largest public protest since thousands took part in sometimes violent anti-Japanese demonstrations in 2005. Police broke up a second protest on Sunday.

A commentary in the city's official Liberation Daily on Friday, written by the Shanghai branch of the Xinhua news agency, said unauthorised protests could cause "social disaster".

"The people have a right to express opinions and suggestions, but equally every person has a duty to make his appeal within a legal framework," it said, without specifically mentioning the anti-maglev demonstrations.

In the city's Minhang district, through which the magnetic levitation train line would run, police have been visiting the homes of protesters and authorities have put up posters detailing laws against unauthorised protest, residents said.

People living by the planned route are opposing an extension of Shanghai's maglev line by 32 kilometres (20 miles). They say electromagnetic radiation could damage their health and complain about the noise, though officials insist the project is safe.

Brown was in Beijing on Friday to discuss bilateral ties and was due to visit Shanghai on Saturday. He said he would bring up human rights and democracy with Chinese leaders, making any crackdown on protesters during his Shanghai visit potentially awkward for authorities.

The demonstrations have also occurred at a sensitive time because a plenary session of the Municipal Peoples Congress, one of Shanghai's biggest annual political events, will open next Thursday and could provide a focal point for protests.

Minhang residents contacted by Reuters on Friday said it was not clear whether protesters would try to stage a repeat this weekend of their demonstrations in downtown Shanghai.

A three-week period for the public to comment on the maglev project via authorised channels - letters and email - ended on Friday with city authorities saying they would study the feedback but giving no sign of whether they might alter the plan.

In an apparent conciliatory gesture, however, the city said residents could keep submitting opinions even after the end of the comment period, which protesters had criticised as too short.

China has the world's only commercial maglev in operation, built by the city government and a German consortium. Costing a reported $1.4 billion and launched in 2003, it reaches speeds above 400 km (250 miles) an hour between Shanghai's international airport and an outlying part of its financial district.