Thousands of students clashed with police in Nanjing following the beating of student vendors by city administration officers.

The demonstration on Monday night came just weeks ahead of the 20th anniversary of the June 4 student democratic movement.

The Hong Kong-based Information Centre for Human Rights and Democracy said yesterday that five student vendors with the Nanjing University of Aeronautics and Astronautics, including one female student, were beaten by officers who were trying to remove them from outside the university's Jiangning campus on Monday evening.

The conflict stirred up lingering anger among students, who have repeatedly complained about heavy-handed treatment by city administration authorities.

The centre said thousands of students marched out of the campus, demonstrating and blocking two main roads at the north entrance.

Photos posted online showed students carrying banners written in Chinese and English. One banner said "Non-violent and Non-Cooperation", seemingly referring to the US civil rights movement.

During the protest three students were taken away by police, which intensified student rage. Protesters clashed with a wall of riot police and smashed a police car. Thirty students were injured, the centre said, adding the protest lasted until 1am in the morning. The fate of the city administration officers is not known.

A student at the university's Minggugong campus said the protests had not spread to his campus.

This was the second time in less than 10 days that university students have protested publicly in the run up to June 4. On May 7, hundreds of students at Zhejiang University peacefully demonstrated along the roads in Hangzhou after one of their classmates, Tan Zhuo , was killed by a speeding driver. The death attracted huge attention among the mainland's online community.

A police officer in Jiangning district confirmed the latest incident yesterday, but denied that police were involved.

The officer, who declined to give his name, said he was not at the campus on Monday night but said it was a small protest. He said police were present to maintain public order but had not clashed with students.

A Nanjing source said the conflict was mainly between city administration officers and students. The source said the student vendors who were beaten might have been from Zhengde College, a subordinate school of NUAA.

Jiangning is a suburban area of Nanjing about 15km from the city centre. According to the source, few Nanjing residents were aware of the demonstration as the city's propaganda department had banned reporting on the event. He added: "Ordinary people are not interested in such stuff nowadays."

"The cause of the conflict was pretty small, but the timing made it extremely sensitive to the authorities who are worrying about anything that might trigger instability among students," he said.