The demonstration eventually attracted tens of thousands of people who besieged city government headquarters, forcing authorities to abandon the project.

'Students are now using instant messaging and social networking websites to call for tens of thousands of people to join the [Qidong] protest,' the centre said. 'Teachers on summer vacation were recalled by schools, and they're now forcing students and their parents to sign letters that guarantee they won't join the 'illegal protest'.'

The organisers reported that most pupils had refused to sign the letter and were still preparing for the rally, the centre said.

Pupils from Qidong Secondary School complained that they were harassed through telephone calls and text messages from authorities and their schools, warning that those who joined the protest would be dismissed from schools and blacklisted by authorities.

'Qidong is hopeless ... the government ignores the public's health and tries to block our protest,' one pupil said. 'Teachers or students who join the protest will be blacklisted, and officials from the city government are now visiting every pupil's home to persuade them not to take to the street.'

Another pupil said she and her parents received at least five calls from the police, warning them against taking part.

A petition widely circulated on the internet said Qidong already had a higher rate of cancer than nearby areas, and a new sewage plant would further destroy its fish stocks.

High school pupils, supported by parents and teachers and mobilised through social media, played a key role in the Shifang protest and this prompted some to ask whether this 'post-90s' generation, as it has been dubbed by the mainland media, will turn up elsewhere to defend its rights