The reported arrest of a former Beijing University student leader has cast a cloud over plans for its centenary celebrations next week. University sources said public security officials had tightened surveillance on campus to avoid trouble during the festivities. Eighteen former students of the university, now dissidents in the US - including Wang Dan and Fang Lizhi - yesterday urged university bosses to press for the release of arrested student leader Wang Youcai, and five former students jailed for their views. 'The atmosphere on the campus is tense despite the approaching festive date,' said one source. 'Student activities in the dormitories, as well as self-organised seminars, are being monitored.' Public security officials were said to be on alert about possible infiltration by troublemakers and were watching for former student activists who wanted to get together with current students. Security guards would only allow special guests and people with invitation cards to the celebrations to enter the campus, one organiser said. Former students needed tickets to join the activities. Another source said more public security officials would be dispatched to the campus this weekend to prevent infiltration. The five other jailed former students are Li Hai, Hu Shigen, Lu Zhizhao, Guo Haifeng and Li Baiguang. The Hong Kong-based Information Centre for Human Rights and Democratic Movement in China said Wang Youcai had been missing since Monday, a day after he travelled to Beijing from his home in Hangzhou, Zhejiang province, to attend Monday's ceremonies. As a Beijing University graduate, he had received a formal invitation to attend the centenary. He was among the 15 most-wanted activists following the 1989 crackdown and was jailed for four years for his role in the protests. He was released on good behaviour in November 1991. A desk clerk at a hotel near the campus, said Wang Youcai vacated his room on Monday after one night. President Jiang Zemin paid tribute to the university when he visited it yesterday. He said: 'The more a country develops, the more it needs talented people. Beijing University has trained many talented people for China's development and I urge you to play an even greater role in the future.'