At face value the clash that took place on 12th June outside the city's legislature complex was just a case of protest turned rough.
4 Days later the civilized world had seen one of the largest mass rally in history. 2 million people, i.e. one quarter of the city's population, reportedly took to the streets in an outcry of refusal to the China Extradition Bill which enables the city government to hand people over to mainland authorities, and that in theory includes transiting tourists within the territory.
The big show down was a direct and immediate consequence of the way Chief Executive Carrie Lam bulldozed the Bill forward. Instead of seeking bipartisan support, Lam has outright called all dissenting comments "BS", including even submissions from the Bar Assoc.
Instead, the Lam administration was more willing to negotiate with the Hong Kong General Chamber of Commerce behind closed doors, and for unknown reasons resolved to raise the threshold for extradition to 7 year prison terms, thereby excluding crimes like pedophilia.
Lam immediately made a public statement afterwards that there was no room for further concession, intending to force the scheme on the city's legislature which was dominated by pro-Beijing puppets.
And this is largely the way things are done after HK's handover back to China. Antecedent to this extradition bill fiasco was the occupy movement in 2014, where the Police Force was also seen using excessive force on protesters.
Andy Tsang, then police chief, addressing the force admist the occupy movement, told the boys they did nothing wrong whilst they were being criticized of using excessive force: https://youtu.be/R8k1-yPt7aY
The ill-managed Force had been defiant and rejected all allegations of siding with their own in investigating the matter, a tradition that sets the scene for the current episode.
Police threatened to open fire upon protesters (occupy movement, 2014)
Triad gangsters were reportedly employed to attack protesters under police acquiescence. Some were even escorted by police officers to leave the scene
Police officers in an assembly chanting "motherf**kers" in unison after the abusive 7 were finally convicted
Tiffany Chen, triad godmother and major donor to police convicts, only refused after public criticism
And this is about how dirty things are going on here.