At least the PLA has had some training in maintaining public order.
In Tiananmen Square they had had demonstrations of that size before but they were orchestrated by the Party. At the end of the day someone would clap theirs hands and everybody went home.
That time they didn't and the police did not know what to do. After a couple of days of trying they gave up and called in the PLA.
The PLA had no experience so they did not know what to do either. Their problem was to clear several thousand people out of the Square.....A couple of battalions of infantry and a squadron of tanks should do it.
That's what they did and the results everybody knows.
Unlikely to happen again unless the HK demonstrators are as pig headed as those in Beijing. Then they had the support of several senior members of the Politburo and could have had their voice heard at the highest level but they persevered in trying to change the entire People's Republic's constitution overnight. With a state visit by Gorbachev due within days the Square had to be cleared, and so it was.