The clashes in the Causeway Bay shopping district followed another night of violent showdowns with police after weeks of pro-democracy unrest in the Chinese-ruled city.
Some shops closed ahead of expected demonstrations, while the MTR metro service said it would cut some services.
Demonstrators chanted anti-government slogans in a New Territories shopping mall as pro-China demonstrators sang the Chinese national anthem on Victoria Peak under a sweltering sun.
About 200 people dressed in red T-shirts gathered on top of the Peak, overlooking the harbor, at around midday. They sang the Chinese national anthem and chanted “I love China”.
Angela, a housewife in her 40s, a Chinese flag sticker plastered on her cheek, said the pro-democracy protesters were “thugs”.
"If the government takes violent action I don’t object,” she said. “We have tolerated enough. I think I have emotional problems because of the riots. Because it’s not safe to go out.”
Lam, the focus of weeks of anti-government unrest, will leave for Beijing on Monday to celebrate the 70th anniversary of the founding of the People's Republic. China's National Day is on Oct. 1.
Lam, who was trapped in an indoor stadium by street protests for hours this week after an "open dialog" with the people, will return to Hong Kong on Tuesday night overland, minimizing the chances of a clash at the airport, a popular target of anti-government protests.
Lam had sent out invitations "requesting the pleasure of your company" at a flag-raising ceremony and National Day reception at the Convention and Exhibition Centre in Wan Chai.
It was not immediately clear why she had the change of heart but the government said she would be deputized by Chief Secretary for Administration Matthew Cheung Kin-chung.
Police fired tear gas and water cannon on Saturday night to disperse protesters who threw petrol bombs and rocks, broke government office windows and blocked a key road near the local headquarters of China's People's Liberation Army.
Anti-government protesters have attacked the legislature, Beijing's main Liaison Office, occupied the airport, thrown petrol bombs at police, vandalized metro stations and set street fires in the former British colony.
Police have responded with tear gas, water cannon, rubber bullets and occasional live rounds fired into the air.
The protesters are angry about what they see as creeping Chinese interference in Hong Kong, which returned to China in 1997 under a "one country, two systems" formula guaranteeing freedoms that are not enjoyed on the mainland.
China dismisses the accusation and has accused foreign governments, including the United States and Britain, of fanning anti-China sentiment.
Protests were sparked in June by planned legislation, since withdrawn, that would have allowed the extradition of suspected criminals to mainland China. But they have since expanded into a broader pro-democracy movement.