The defendants, aged between 20 and 29, had denied the charges.
Judge Josiah Lam said there was a riot on Gascoigne Road moments before their arrests outside Diocesan Girls’ School, and protesters had thrown Molotov cocktails towards a police cordon.
The judge said the defendants had deliberately geared themselves up for a riot, adding that they had ignored police warnings and stayed at the scene, thereby encouraging others to engage in violence.
Two of the defendants were also found guilty of possessing offensive weapons or items fit for unlawful purposes, namely a pair of scissors, spanners, and a can of lighter fuel.
Sentencing was adjourned until January 15 and all nine were remanded in custody.