SHANGHAI, Jan 13 (Reuters) - Police broke up a demonstration against a planned extension of Shanghai's high-speed "maglev" train line on Sunday, pushing and dragging dozens of protesters off one of the city's most crowded shopping streets.

Officers cordoned off part of Nanjing Road as they prevented a repeat of Saturday's demonstration there by hundreds of people, Shanghai's largest public protest since thousands took part in sometimes violent anti-Japanese demonstrations in 2005.

Uniformed and plainclothes police chased and struggled with some of the protesters, shouting at a Reuters reporter on the scene not to talk to Chinese bystanders.

Earlier, chanting white and blue collar workers, many with homes next to the planned route of the magnetic levitation train, assembled to demand that city authorities reconsider the project because it could damage residents' health.

They said they feared electromagnetic radiation from the line would sicken them, and also complained about the noise.

"The government obviously doesn't care about our welfare," said a 29-year-old protester surnamed Huang, who described himself as self-employed.

China has the only commercial maglev in operation in the world, developed and built by the government and a German consortium including industrial giant Siemens <SIEGn.DE>.

Launched in 2003, the maglev reaches speeds of over 400 kilometres (250 miles) an hour as it floats on a magnetic cushion between Shanghai's international airport and an outlying part of its financial district in Pudong, 30 km (19 miles) away.

The government wants to extend the line by 32 km (20 miles) through Shanghai to near the city's domestic airport, and then possibly to the tourist city of Hangzhou 200 km (125 miles) away.

Government officials insist the project is safe and have given the public until this Friday to comment on the proposed route of the first stage of the extension, which has been posted on a city government Web site.

But protesters say the comment period, which began on Dec. 29, is not long enough and the process is not transparent. Along parts of the proposed route, posters denouncing the project have been displayed on apartment buildings, an uncommon sight in a country where unauthorised public protest remains illegal.

Nationwide, Chinese authorities face a rise in protests over environmental issues, corruption and abuse of official power. But major protests in large cities such as Shanghai, which could become more politically sensitive, are rarely allowed to happen. (Reporting by Edmund Klamann and Royston Chan; Writing by Andrew Torchia; Editing by Jerry Norton)

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