The police also used water cannons on Sunday after protesters vandalized a subway station and hurled bricks and gasoline bombs at a complex of government buildings that includes the city’s legislature, during a weekend that revealed the extent to which three months of pro-democracy demonstrations have frayed the city’s social fabric.

The South China Morning Post reported that at least one man who was attacked by a mob of black-clad protesters on Sunday was in serious condition. Video footage showed several men being taken away on stretchers or treated by paramedics after an evening of fistfights and street brawls between people on opposing sides of the city’s yawning political divide.

The turnout at the march on Sunday was lower than that of similar ones this summer, but the violence over the weekend highlighted the staying power and raw anger of a movement that has produced 15 consecutive weekends of unrest in an otherwise orderly financial hub.

The tumult across the city came just over two weeks before a major political moment on Oct. 1: the 70th anniversary of the founding of modern China. A key question is what protesters will do on that date, and how Beijing and the Hong Kong police will respond.

“I don’t think the government will be able to respond to our demands by Oct. 1, so people will keep fighting for what they want,” Cheng Sui-ting, 27, an environmental educator, said at Sunday’s march, which began in the Causeway Bay shopping district and quickly stopped traffic.

Carrie Lam, Hong Kong’s beleaguered leader, said early this month that she would formally withdraw the contentious extradition bill that prompted the initial protests in June and led to the territory’s worst political crisis since it returned to Chinese control in 1997.

But mass rallies have continued, in part because the movement’s demands have gradually expanded to include broad calls for political reform, including universal suffrage, and an independent inquiry into allegations of police brutality.

Early Sunday afternoon, protesters began marching west from the shopping district of Causeway Bay toward Admiralty, an area that includes many government offices and the Hong Kong headquarters of the Chinese military. Some confronted police officers who were stationed on a footbridge near the police headquarters. “Corrupt cops,” they chanted, “may your whole family die!”

Others occupied a major road in Admiralty, piling traffic barricades inside a nearby train station and across one of its entrances. A few used metal poles and umbrellas to smash some of the station’s glass railings.

The police responded by blanketing the streets with tear gas, spraying arcs of blue-dyed water from cannons and deploying riot police officers to chase protesters from the area. Many demonstrators headed eastward, where they built more barricades and set a fire outside the Wan Chai subway station, between Causeway Bay and Admiralty.

Chris Cheung, a 22-year-old student who was helping to reinforce a makeshift barricade an exit to Admiralty Station, said that protesters were deliberately targeting the city’s subway operator, the MTR Corporation, for having allowed the police to beat protesters inside a station in late August.

If Hong Kong citizens “have a conscience,” he added, “they would have watched the news and seen how people got beaten and understood our perspective.”

In the past three months, protesters have been tear gassed by the police and attacked by gangs of men widely thought to be linked to organized crime syndicates. Yet by plunging into vandalism and street brawls, they risk squandering some of the wide support they receive from the Hong Kong public.

Ray Siu, a 33-year-old construction worker who has joined the protests, said he felt that while attacking the MTR was acceptable, attacking shops or people at random would bring a different reaction.

“We haven’t crossed that line yet,” he added.

The weekend started with fistfights on Saturday in at least two areas of Hong Kong, apparently after protesters confronted government supporters who were vandalizing the so-called Lennon Walls that the movement has set up across the city for pro-democracy messages and artwork.

At one point on Saturday, scuffling broke out in the Fortress Hill neighborhood of Hong Kong Island near the site of a vandalized Lennon Wall. Video footage showed men using Chinese flags to beat other men, presumably pro-democracy demonstrators, in the middle of a busy road.

The Hong Kong Hospital Authority said that at least 25 people had been hospitalized with injuries sustained in scuffles on Saturday.

New clashes between civilians erupted Sunday night in Fortress Hill and nearby North Point. Footage showed two bloodied men being lifted onto stretchers by paramedics.

Two witnesses in North Point said they had seen a group of men carrying long knives and pouring gasoline onto the street as police officers pursued them. The men retreated to a building, where local outlets reported that gangsters who attacked protesters had fled.

The march on Sunday afternoon came a week after a rally outside the United States Consulate descended into vandalism, and followed days of smaller pro-democracy rallies around the city.

Days before Sunday’s march, an established Hong Kong advocacy group, the Civil Human Rights Front, applied for a permit to hold the march legally. The police rejected the request, citing concerns about public safety, and the group officially canceled the march.

But many protesters defied the police by marching anyway, as they have done before.

“I come out because it is my right, and I don’t care whether the police agree or not,” said Joesy Lau, 53, a clerk at an investment firm. “It’s my right.”