Four hours later, most of the protesters had vanished, allowing Lam and her government ministers to leave the stadium through the back door on Queen’s Road East at 1.30am on Friday.

The police source said the force had studied different routes to escort Lam and the four principal officials from the venue in an emergency.

During their confinement, Lam and her ministers reviewed the session and studied possible improvements in future dialogues.

In the unprecedented event at the stadium, many attendees, as candid as they were critical, grilled Lam on why, after making a U-turn to announce the formal withdrawal of the extradition bill that sparked the crisis, she would not give in to protesters’ remaining demands – the most overwhelming being an independent inquiry into allegations of police brutality.

A government source said the decision for Lam and her ministers to stay behind was made at high-level cross-departmental meetings in preparation for the dialogue session.

“Police kept a minimal presence to set participants of the dialogue at ease. They adopted a reactive approach on Thursday,” the source said.

“They didn’t take action even when protesters dug up bricks outside.”

Another government source familiar with the town hall dialogue said: “The chief executive was prepared to be stuck inside the stadium for some time to avoid confrontation.

“The public perception would be very bad if clashes between police and protesters had erupted outside the stadium soon after the dialogue which had been conducted in a peaceful manner,” the source added.

Speaking after a press conference on the government’s relief measures for the tourism sector on Friday, Secretary for Commerce and Economic Development Edward Yau Tang-wah, one of the ministers present at Thursday’s dialogue, confirmed officials deliberately stayed behind to avoid sparking clashes.

“Although there were people waiting, some taking bricks and blocking roads, we believed we were having a peaceful, heart-to-heart talk,” he said.

As it happened: Carrie Lam holds public dialogue to address protests in Hong Kong, but remains in venue for four more hours

Government officials having a candid and open conversation with the public would help “create a conducive environment for more conversations”, Yau added.

He said Thursday’s dialogue was just one form of communication with different sectors of the society, explaining why officials did not talk to protesters outside the stadium.

“Last night’s meeting showed many people are happy and willing to talk to the government. We’ll have further considerations about the format of the next conversation,” he said.

The Post understands the government had considered up to six options for the venue to host Thursday’s event.

“Queen Elizabeth Stadium was chosen after taking into account factors such as convenience and distance from densely populated neighbourhoods,” a source familiar with the matter said.

Chance to pin down Hong Kong leader Carrie Lam on protest chaos came with few expectations – and just as few new answers

Kong Wing-cheung, senior superintendent from the force’s public relations branch, said the security arrangement for events attended by the chief executive were considered confidential.

“We carried out stringent risk assessments about whether any public events would be held in Wan Chai or near the venue,” he said.

“Last night there were some organisations and people who held public events or protests in the vicinity of the stadium without notifying us,” Kong said. “We have to ensure we have sufficient manpower to deal with any sudden event nearby.”

Additional reporting by Fiona Sun