It would be true that some protesters marched with violence. But, I don't know if it's the intention of the leaders who hosted the movement.
So, I agree with arrest in terms of investigating the intention of violent activities. If Chinese Communist Party ignored such an investigation and put them into prison, I would criticize the party.
And, it's not important for CCP to judge if it had violence or not. It's important from them to understand how serious it is for Hong Kongers to live without democracy.