I from mainland, just here to say, people's opinion is effected by the information they got, and they may just don't have enough information.

I'm is still learning about this series of events happened in both Hongkong and China mainland, so I can't provide a conclusion just yet. But many mainland people are supportive towards Hongkong protesters. Example: see comments below 1 and 2 to name a few.

My current general opinion (Without reach a conclusion about this event) are:

• I don't support Hongkong independent movement. And to my understanding, they're not very popular anyways.
• I against use of force against peaceful protesters. I believe it is the government's responsibility to keep things claim without hurting it's people, and I believe is the government's fault if they've failed to do so.

^{I'll delete this comment few hours later. I know some subreddits are heavily monitored by China, and I don't want to pay a visit to prison.}