HONG KONG — Tens of thousands of demonstrators marched across Hong Kong’s main island on Sunday to protest the city’s embattled government, occupying major downtown thoroughfares in defiance of a police directive that had attempted to limit their movements.
The demonstration was focused largely on the protesters’ demand for an independent investigation into what they said was police brutality in earlier street clashes. And as night fell and the crowds of protesters swelled, yet another clash with the police seemed likely, if not inevitable.
The conduct of Hong Kong’s police force has come into sharp focus in recent weeks as some of the city’s largest-ever demonstrations have thrown it into the worst political crisis since China reclaimed sovereignty from Britain in 1997. The protesters accuse the police force — which has long been known in the region as “Asia’s finest” — of using excessively violent tactics to suppress their gatherings.
The demonstration was the latest sign of growing antagonism between the largely peaceful protest movement and the front-line officers patrolling it.
On Friday, the police said, officers raided an industrial building and seized about two pounds of powerful explosives, 10 gasoline bombs and nitric acid, as well as bullets, slingshots, knives and metal rods.
The police described the site as a “homemade laboratory” of triacetone triperoxide, a highly unstable explosive also known as TATP. They said they were investigating whether the explosives were related to the Sunday protest but did not have adequate evidence to make conclusions.
Three men in their 20s were arrested in connection with the case.
On Sunday afternoon, protesters dressed mostly in black T-shirts set off from Victoria Park in the Causeway Bay area of Hong Kong’s main island. Some carried signs saying, “No extradition to China” and “Stop police brutality.”
The mass demonstrations began in early June in response to unpopular legislation that would have allowed extraditions to mainland China, where the courts are controlled by the governing Communist Party. The bill has since been suspended but not fully withdrawn, one of the protesters’ key demands.
“The government must withdraw the bill and set up an independent inquiry committee to investigate the police,” Tommy Tsang, an 81-year-old retiree, said at the protest on Sunday.
He said he was particularly angered by the police violence. “If you don’t hit people, why would they hit you back?” he said.
Supporters of the push for an independent inquiry include members of the city’s pro-democracy legislative minority, the Hong Kong Bar Association and the European Parliament, which issued a statement on Thursday that called for an “independent and impartial investigation into the use of force by the Hong Kong police against the crowds.”
An open letter signed by a few hundred people who said they were relatives of police officers has also called for an independent inquiry.
But advisers to the territory’s embattled chief executive, Carrie Lam, said that her administration did not intend to make further concessions to the protesters. That stance suggests the government is confident it can weather further protests despite signs that the unrest could damage the local economy and the risk that more protests could result in injuries or deaths among demonstrators or police officers.
Police officials say that they have largely acted with restraint and have used force only when attacked by protesters. They accused some protesters of rioting during recent demonstrations, including one in which a small group forcefully stormed the Hong Kong legislature and spray-painted political slogans.
The police and a watchdog that monitors complaints against them have said they plan to investigate the tactics used against protesters at a June 12 demonstration that turned violent. Many people in Hong Kong, a city of about seven million, say they believe that the police response that day — which included firing tear gas, rubber bullets and beanbag rounds — was excessive.
But the protesters and their supporters argue that the watchdog is not independent and will not conduct a fair investigation. They also note that the Hong Kong government has commissioned past inquiries over civil unrest.
The protesters at the march on Sunday spanned a diverse age range. “Everyone is out here for their children,” said Sam Tam, 57, who attended marches this month with his 8-year-old daughter and about 18 relatives.
“The older generation has to say sorry to the younger generation for not listening to their voices,” he added.
Brian Chan, a protester in his late 40s, waved an American flag as he marched. “Like many protesters, we want Trump to liberate Hong Kong, and to pass laws that will help the democratization of Hong Kong,” he said. “We need help.”
Before the march, protest organizers had requested a police permit to march west from Victoria Park to the Court of Final Appeal, in the center of Hong Kong’s primary business district. The police denied that request and said the march would have to officially end in the city’s Wan Chai district, just east of Admiralty, a district that houses the city’s legislature and other government offices.
But on Sunday afternoon, thousands of protesters at the front of the crowd defied those orders by streaming west into Admiralty, where the police had erected 6.5-foot plastic barriers outside parts of the government office complex.
Some of the protesters there approached a metal fence outside the complex and began heckling police officers in riot gear who were stationed inside. Others gathered outside the Hong Kong Police Headquarters nearby, chanting, “Shame.”
By early evening, many protesters had passed the Court of Final Appeal in Central Hong Kong. They appeared to be heading toward the Chinese government’s liaison office in the city, which lies farther west on the island and where a crowd was already gathering. Some of the protesters partly blocked a nearby road with road signs and barriers.
The protest on Sunday came a week after a rally urging Mrs. Lam to resign and fully withdraw the extradition bill devolved into a frenzied brawl between a small group of protesters and officers in riot gear in a luxury shopping mall.
The protesters have also turned their anger at mainland Chinese who visit Hong Kong to shop or to buy medicine, powdered baby formula and other goods for resale across the border. Last weekend, a demonstration in a Hong Kong border town against so-called parallel traders ended in clashes with the police.
Afterward, the Junior Police Officers’ Association, a union, called on the Police Department to safeguard the personal safety and “emotional well-being” of officers in the field.
That statement came days after Mrs. Lam condemned last weekend’s street clashes and thanked the city’s police officers for “safeguarding Hong Kong’s safety at the front lines.”
Demonstrations in support of the police have also been organized by pro-establishment groups, including one on Saturday that organizers said drew more than 300,000 people. The police put attendance at just over 100,000.