Students from Nanjing University of Aeronautics and Astronautics took to the streets late Monday after local government security guards allegedly beat several students, the Information Centre for Human Rights and Democracy said.

"Several thousand students began demonstrating and blocking the street," the Hong Kong-based rights group said in a statement late Tuesday.

"The college students clashed with riot police, resulting in injuries to 30 students. One police car was smashed by protesters."

At least five students were detained by police as the protests continued early Tuesday morning, it said.

The demonstration erupted after city security guards tried to stop a handful of students from peddling items at an unregistered night market in front of the university, it said.

Several of the students were beaten and roughed up, it said.

Officials in Nanjing's Jiangning district government, where the university is located, refused to comment on the unrest when contacted by AFP, as did local police.

"None of this happened at all," a security official at the university told AFP by phone.

"No students were detained, beaten, or injured. There were no clashes between students and security guards or police."

Photos of the unrest posted on the Internet showed hundreds of people gathered in an intersection blocking traffic, some holding signs calling for an apology from the security guards.

Pictures of ordinary policemen facing down protesters were also shown, as were images of a row of fully equipped riot police.

The unrest occurred ahead of the 20th anniversary of the bloody June 4 crushing of the Tiananmen democracy protests, when authorities called in the military to quell six weeks of peaceful protests in central Beijing.