Police fire tear gas as Hong Kong protesters rally against national security law

11 and a half months ago, 2 million marched peacefully. 3 days later they started firing tear gas.

I know it is a "domestic" issue for China, but surely other countries can't standby and watch this happen.

And no, it isn't. This is CCP propaganda. Our autonomy is guaranteed by an international agreement, the Sino-British Joint Declaration. We were guaranteed 50 years, unchanged, under One Country Two Systems. This isn't a domestic issue. This is the CCP violating (yet) an(other) international treaty.