The biggest clashes took place in or near Mass Transit Railway (MTR) stations because stations are often closed at the government's behest to stop demonstrators from gathering
Hong Kong cleaned up on Monday and resumed train services after a weekend of sometimes violent protests that saw pro-democracy activists vandalise a railway station and shopping mall.
Police on Sunday fired tear gas to disperse protesters in the latest clashes in more than three months of unrest that has plunged the Chinese-ruled city into its worst political crisis in decades.
The biggest of several clashes took place in or near Mass Transit Railway (MTR) stations, now a familiar target of attack because stations are often closed at the government's behest to stop demonstrators from gathering.
Hundreds of protesters had gathered in the New Town Plaza in the New Territories town of Sha Tin on Sunday, chanting: "Fight for freedom" and "Liberate Hong Kong."
Activists trampled on a Chinese flag near the train station and rounded on a man they believed had opposed them. Protesters also smashed video cameras and ticket booths in the station.
Some started to trash fittings at the entrance of the mall. The protesters then spilled outside where they set fire to barricades made of cardboard, broken palm trees and other debris.
MTR said on Monday train services had returned to normal.
The former British colony is on edge ahead of the 70th anniversary of the founding of the People's Republic on Oct. 1, with authorities eager to avoid scenes that could embarrass the central government in Beijing.
The Hong Kong government has already called off a big fireworks display to mark the day in case of further clashes.
China, which has a People's Liberation Army garrison in Hong Kong, has said it has faith in Hong Kong leader Carrie Lam to solve the crisis.
Demonstrators are frustrated at what they see as Beijing's tightening grip over the Asian financial hub, which returned to China in 1997 under a "one country, two systems" formula intended to guarantee freedoms that are not enjoyed on the mainland.
China says it is committed to the "one country, two systems" arrangement and denies interfering.
Hong Kong also marks the fifth anniversary this weekend of the start of the "Umbrella Movement" pro-democracy protests that failed to wrestle concessions from Beijing.
Anti-government protesters, many masked and wearing black, have caused havoc since June, throwing petrol bombs at police, trashing metro stations, blocking airport roads and lighting street fires.