The University of Hong Kong vice-chancellor is under pressure to attend an open discussion with students over much-criticised security arrangements for the visit of Vice-Premier Li Keqiang to the campus last week. A full-page advertisement signed by 1,500 people - most of them HKU students and alumni - in a Chinese-language newspaper today calls on Professor Tsui Lap-chee to apologise and for Police Commissioner Andy Tsang Wai-hung to resign. 'By inviting [the vice-premier] to attend the ceremony celebrating the university's centenary, the university has demeaned itself from a free campus to a place suppressing dissidents,' the statement says. More than 7,000 people have also joined a Facebook group to condemn the university's reaction to the row and to state that the incident has undermined freedom of expression. They also demanded that Tsui, who has since criticised police action during Li's visit, attend an open forum on the campus on Friday to explain how HKU had invited the vice-premier and how it had decided on the security. Li was in the city for three days. On Thursday, he went to HKU, where protesting students were hemmed in by police well away from the celebration venue. Some of the students were dragged to the ground and a third-year politics undergraduate, Samuel Li Shing-hong, was detained for an hour. A university spokesman later said the police were mainly responsible for the security set-up and that HKU only provided help. Tsui also attended a forum last Friday to explain the arrangements and offered an apology to Samuel Li. But HKU was criticised for calling the forum at short notice and failing to give a clear explanation. A day later, Tsui issued another statement saying the 'strength and measures of handling the protesters by police was unacceptable'. A spokeswoman said yesterday that HKU would provide a venue for the upcoming forum but had not yet decided who would represent it. Former student and forum organiser Carman Chan Wai-man said she hoped Tsui would attend the event to give a clearer explanation and to gain a better understanding of how students expected him to safeguard freedom of expression at HKU. Samuel Li said Tsui had asked him to a private meeting, but he had declined the invitation as he expected the professor to clear the air in public. Secretary for Security Ambrose Lee Siu-kwong said in Beijing that he was aware of the security criticisms. He said the police would review the measures and give an account at a special legislature meeting on Friday. Deputy police commissioner John Lee Ka-chiu reiterated the force respected freedom of expression but at the same time had a duty to ensure the safety of visiting state leaders.