In general there shouldn't be a problem. For e.g. during the historic march this sunday when 1.7 million walked from CWB to Central, the MTR was jammed on Hong Kong island and roads blocked for a few hours but after that things went back to normal.
There are a few marches and rallies planned until end aug and there would be more (I assume) till the situation has been resolved. As bro @twiceaweek said things are relatively safe as long as you don't venture too close to events where riot police and protestors are in a face off.