HONG KONG - Hundreds of thousands of protesters packed the streets of Hong Kong on Sunday to mark six months of an unprecedented, sweeping anti-government movement in this Chinese-ruled, semi-autonomous city.

The police-sanctioned march on Sunday which started at 3pm local time was largely peaceful but tensions escalated in the evening, when riot police got locked into a tense standoff with a large group of black-clad protesters who split off from the authorized rally to occupy a major thoroughfare in the business district of Central, the end point of the protest route.

Protesters set up makeshift barricades with plastic roadside barriers, metal sheets, bamboo poles and other objects on the thoroughfare and a box marked with the message "Do not kick, it may explode" was placed on the road.

Some riot police officers pointed their non-lethal shotguns at people and journalists gathered there and ordered them to leave. The police's water-cannon-equipped anti-riot vehicle also stood by.

The protest movement, sparked by a controversial extradition bill which could see individuals sent to mainland China for trial, started with a mass demonstration attended by around a million protesters on June 9, but it has since morphed into a broader and increasingly violent movement.

Demonstrators wearing protective gear on Sunday also set up makeshift road blocks at other locations along the protest route. Police warned protesters that "necessary action" would be taken if protesters ignore instructions to disperse, noting that the protesters had gone beyond the end point of the protest route.

Police issued a statement saying a group of protesters vandalized shops and a bank during the rally and warned them to refrain from "illegal acts posing a threat to public order and endangering public safety." A Chinese-owned bank was smashed up with broken glass littering the floor and sign was placed outside the bank that says "Love China, hate the party", reported public broadcaster RTHK. A Starbucks cafe, run by a franchise company seen as pro-Beijing, was also vandalized.

Earlier in the evening, the fire alarm was set off at the High Court and broken bottles were found at its front entrance, which had been burned black. The message "Rule of law is dead" was emblazoned on the wall of the building.

The organizer of the protest, Civil Human Rights Front, said around 800,000 people participated in the march on Sunday. The participants in the authorized rally came from a vast age range and backgrounds, from parents pushing toddlers in strollers, young people, middle-aged professionals to pensioners. Some were in wheelchairs.

It was the first police-sanctioned mass protest for almost four months and was also the first after pro-democracy politicians scored a landslide victory in a district election last month.

Unlike many of the recent protests which had been banned by police, the march on Sunday had a relaxed atmosphere and protesters were in high spirits.

"Good guys don't become police!" yelled protesters at riot police officers guarding a footbridge, while many stuck their middle fingers at them.

Earlier in the day, the unofficial anthem of the movement wafted in the air as people chanted "Five demands, not one less!", referring to the political demands yet unfulfilled which included universal suffrage and an independent investigation into police brutality.

The extradition bill which sparked the protest in June was belatedly scrapped in September but many ordinary Hong Kongers say excessive police force should be investigated by an independent body.

Reverend Chu Yiu-ming, raising funds for victims of police brutality in the six-month movement, said to a crowd: "We want our freedoms back so our young people can regain the freedom from fear."

"I feel so sorry for our young people the police have real weapons and bullets, while the youngsters have only bricks and Molotov cocktails. They have no other way to resist," said Mary Tse, a retiree.

A young couple was seen waving a giant U.S. flag during the protest Sunday. They said they were grateful to the United States for passing the Hong Kong Human Rights and Democracy Act.

"I am truly thank to the U.S. for this law. There is nothing more we can do to change our political system and if we don't come out to fight there will never be an opportunity," said Joe Lai, 30.

Earlier in the day, police said they had seized a semi-automatic Glock pistol and 105 bullets in an operation. Police arrested eleven people aged between 20 and 63.