A US State Department spokesman said: "We urge all sides to exercise restraint and refrain from violence. It is important for the Hong Kong Government to respect the freedoms of speech and assembly, as guaranteed by the Basic Law.
"Societies are best served when diverse political views are respected and can be freely expressed. The ongoing demonstrations in Hong Kong reflect the sentiment of Hong Kong’s people and their broad concerns about the erosion of Hong Kong’s autonomy.
The spokesman said reports of violence by criminal gangs were "disturbing".
"Reports of organised violence by criminal gangs against private citizens, and attacks on journalists trying to do their jobs, are particularly disturbing. The continued erosion of Hong Kong’s autonomy puts at risk its long-established special status in international affairs."