HONG KONG (Reuters) - Hong Kong riot police fired tear gas late on Monday to disperse pro-democracy demonstrators gathered to commemorate the three-month anniversary of an assault by more than 100 men on protesters, commuters and journalists.

The clashes in the Yuen Long neighbourhood came a day after widespread violence in which tens of thousands marched through Kowloon district and hardcore activists threw petrol bombs at police, torched metro entrances and trashed shops.

Hong Kong has been battered by five months of huge and often violent protests over fears Beijing is tightening its grip on the territory, the worst political crisis since colonial ruler Britain handed it back to China in 1997.

Under a policy that deems marches illegal unless they have a police permit, riot police stopped around 100 protesters reaching the Yuen Long metro station in Hong Kong's northwest, which was closed five hours early amid tight security.

Police ordered protesters to disperse, at one stage rushing them and detaining one person. Scuffles broke out between pro-Beijing supporters and protesters and angry residents emerged from apartments to jeer officers, calling them "black police".

A police statement said police resorted to tear gas after some protesters hurled "hard objects" at them and vandalised bank facilities in the vicinity. It urged residents to stay indoors, keep windows closed and avoid local streets.

After a few hours most protesters had scattered but police remained on the streets in force, occasionally firing tear gas at small groups and chasing down individuals. A few masked protesters taunted police but quickly disappeared up side streets when officers approached.

Elsewhere in the city, protesters staged peaceful sit-ins at five metro stations.

JULY MOB ATTACK ON PROTESTERS

Protesters are angry that police did not act quickly enough to protect pro-democracy activists and commuters from the July 21 gang assault at the Yuen Long metro, and at what they say is a slow investigation into the incident.

Police have arrested 34 people and charged six.

At the time some believed the men had been hired to attack the group. Some politicians and activists have linked Hong Kong’s shadowy network of triad criminal gangs to political intimidation and violence in recent years, sometimes against pro-democracy activists and critics of Beijing.

One of Monday's protesters, a secondary school pupil, said he could not accept a call from the government to move on and wait for a report from the Independent Police Complaints Council into the Yuen Long attack.

"We can't forget this, because I think that this is so horrible to all Hong Kong people, and this should not be moved on from until the criminals are caught and in the jails," he said, without giving his name.

Yim, a 42-year-old social worker, said he was in Yuen Long station on the night of the July attack, adding it was important to continue to protest against such mob violence.

"There were many people filmed that night but they’ve only arrested six people so far ... we want an independent investigation and justice for this attack."

Many in Hong Kong feel police have used excessive force against protesters and want an independent investigation into police actions.

LAM APOLOGISES TO ISLAMIC COMMUNITY

After two weeks of relative calm, Sunday's massive rally showed there was still widespread support for the pro-democracy movement and no end in sight to the unrest in the Asian financial hub, which is facing its first recession in a decade.

Embattled Hong Kong chief executive Carrie Lam has rejected the demands of protesters and backed police actions.

Lam apologised to the city's Muslim community on Monday after a police water cannon fired blue dye at a major mosque during operations on Sunday night. Police said the action was an accident and also apologised, but the incident has heightened racial tensions, with Islamic leaders calling for calm.

There have been few major rallies in recent weeks, but violence has escalated at those held, with militant activists setting metro stations ablaze and smashing up scores of shops, often targeting Chinese banks and stores with mainland links.

Police have fired thousands of rounds of tear gas, hundreds of rubber bullets and three live rounds at brick- and petrol bomb-throwing activists.

Since the protests escalated in June, more than 2,600 people have been arrested, many under 18 years of age, while two people have been shot and many more injured.

Many people in Hong Kong are angry at what they see as mainland China's attempts to limit the freedoms the city enjoys under the "one country, two systems" principle enshrined in its handover from Britain in 1997.

The protests pose the biggest popular challenge to Chinese President Xi Jinping since he took power. Beijing has denied eroding Hong Kong's freedoms and Xi has vowed to suppress any attempt to split China.

Protesters are demanding universal suffrage, an independent inquiry into alleged police brutality, amnesty for those charged, an end to labelling protesters as rioters, as well as the formal withdrawal of a China extradition bill that was the original trigger for the unrest.

(Additional reporting by Twinnie Siu; Writing by Michael Perry; Editing by Mark Heinrich)