Maybe, the Tienanmen Square riots in Beijing several decades ago ended up that way. And reports are that the Muslim riots in Xinjiang Provence have been bloody.
I doubt very much that the riots will spill onto the mainland. Frankly many if not most Chinese are pleased with their government and their new economic status in the world. And they see democracy as anarchy and an impediment to economic progress since China is catching up to the US.
HK is distinctly unique and different from the Chinese mainland. It spent centuries as a British colony, China did not. Hong Kong spent decades as a democracy after the Brits pulled out. China did not.
If you went to HK today, you'd swear that you were in a large shopping mall in the United States. A China town in the US to be sure, but the US nonetheless.
Although mainland China has become very westernized, it's not that westernized. So, no, the riots will not spread into the mainland.
Finally, no the US will not intervene. There would be nothing to gain and everything to lose by starting a war with Beijing.