Police have arrested around 1,200 people related to the protests since June.
Thousands of Chinese military personnel paraded in Shenzhen, just across the border from Hong Kong.
Protesters marched to the U.S. consulate as leading activist Joshua Wong travelled to Germany to rally support - prompting an angry reaction from China - and then to the US.
Hospital authorities said eight people were injured throughout the day, including three in serious condition.
The hospital rally came after a pro-democracy march on Sunday in downtown Hong Kong descended into chaos.
The protesters want their demands for greater democracy and police accountability met, and some of them defend violence as necessary since peaceful demonstrations haven't effected change. Rail operator MTR Corp has become a prime target of vandalism, with activists angry that it closes stations during protests and prevents demonstrators from gathering.
The marchers burned Chinese flags and tore down pro-government posters, smashing windows and hurling Molotov cocktails at police - who fired tear gas and water cannons at the crowd. They fled again after riot police advanced and the cat-and-mouse battles went on for a few hours before calm returned.
Pro-Beijing supporters turned up at the North Point and Fortress Hill neighborhoods late Sunday, leading to brawls.
"Whenever rioters prepare to hurl petrol bombs, officers on the field could very likely interpret that as a life-threatening attack against themselves or others and respond with relevant use of force or weapons, including firearms with real bullets", he said.
Referring to incidents at the MTR stations and elsewhere, police in a statement on Monday condemned "violent acts which disregarded law and order" carried out by protesters. The police arrived some 40 minutes later despite there being a police station nearby. Tse said the mob also assaulted pro-government supporters, including a man who was beaten until he was unconscious.
Almost 1400 people have been arrested since the protests started in June, but police gave no update on the number arrested over the weekend. The global airport handled 6 million passengers in August, down 12.4% from a year earlier, according to figures published by the Airport Authority on Sunday.
The government in a statement late on Sunday said violence would only harm the community and it was sincerely trying to solve problems.
Hong Kong's airport cancels flights on August 12 after being invaded by thousands of black-clad protesters.
The unrest has battered Hong Kong's economy, which was already reeling from the US-China trade war.
But since the recent unrest began, China has accused Britain of being behind the protests.
And they may only get more intense in the run-up to October 1, when China celebrates 70 years of Communist Party rule.
The Civil Human Rights Front has also called for a mass rally at Victoria Park, simply to the east of the central business district, but authorities have denied consent due to earlier clashes after huge parties.
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