Dozens of University of Hong Kong students who want its president to condemn police for their handling of protests surrounded his office on Friday demanding open talks before a smaller group of radicals targeted his home, a day after threatening to step up their campaign.

Zhang Xiang was criticised by some students this week for swerving their calls to speak out directly against alleged police brutality and instead voicing opposition to “any form of violence by any party”.

In a letter issued on Friday morning, the university head responded to a second ultimatum by saying he hoped to take part in a forum with students this month.

The first ultimatum was a petition signed by over 3,500 students, staff and alumni for Zhang to respond by October 28 to their four demands, which include issuing an open letter condemning police and holding an open dialogue with students.

Responding to that on Monday, Zhang did not touch on the forum or single out police in his condemnation of violence.

Discontented students set a new Thursday deadline for Zhang to improve his response or they would escalate their campaign.

Following his second response on Friday morning, more than 50 students, many black-clad and wearing masks, joined a sit-in outside Zhang’s office that afternoon, demanding he specified a date for a “sincere” forum.

A petition letter was received on Friday by Vice-President Ian Holliday, with Zhang not in the city.

An HKU student and organiser of the petition, who gave his name as “Blue”, said students were disappointed with Zhang’s delayed response, and his failure to say when a forum would be held and if it would be open-door.

“The university had repeatedly ignored our concerns ... We hope next time when we submit a [petition] letter we hope to meet Zhang as president of HKU,” he said.

During a half-hour meeting on Friday some students raised concerns with Holliday about police entering campus without prior notice and management’s approach to supporting arrested students.

In response, Holliday said the university has a “clear protocol” about police conducting searches on campus, and the institution had been in “constant dialogue with students”. He added they were actively scheduling a date for the forum.

More than 10 students later moved onto Zhang’s residence, where they sprayed graffiti reading “details of forum now” on the road outside, placed objects including a rubbish bin in front of the gates and threw three traffic cones into the property.

University heads have been under pressure to follow the example of Chinese University president Rocky Tuan Sung-chi, who responded to student demands by issuing an open letter last month condemning “proven cases” of police violence.

Meanwhile, about 100 students from Hong Kong University of Science and Technology staged a rally at about 1pm on the campus to protest against the university’s change of venue for the graduation ceremony without consulting students, the government’s mask ban and what they called police violence during protests.

Hong Kong has been roiled by protests since June, sparked by the now-withdrawn extradition bill which would have allowed the transfer of criminal suspects to mainland China. The demonstrations have morphed into a wider anti-government movement seeking genuine universal suffrage, among other demands.

The HKUST students also raged against university management for not allowing their representative Fu Pui-ying to take office at the university council this year.

The crowd marched up to the offices of the president and vice-president at around 1.30pm, but left after 15 minutes when the president did not appear.

At around 2pm, HKUST vice-president of administration and business Mark Hodgson met the students to talk about the student representative issue.

“The university is investigating the complaints it has received ... I can confirm that we will release the results as soon as we can,” said Hodgson.