HONG KONG — Popular central Hong Kong neighborhoods became foggy battlegrounds Sunday as police for the second day in a row fired tear gas and projectiles at thousands of protesters who were again demonstrating without official authorization.

The clouds of gas left bystanders — including children, tourists and the elderly — choking and sputtering, underscoring the growing risks of Hong Kong’s deepening political crisis, now in its eighth weekend. Police said that protesters threw paint bombs, corrosive liquid and bricks at them, and that 49 people were arrested.

Protesters had convened to demonstrate against what they saw as police brutality over the course of the protests, including in the neighborhood of Yuen Long the day before. They marched across a nearly four-mile stretch and clashed with police at several points, such as at the iconic Causeway Bay shopping district.

By 7 p.m., police had started firing round after round of tear gas to clear protesters.

Police and protesters alike shouted warnings to residents to close their windows and protect themselves from tear gas. But the noxious gas fired around the trendy Sheung Wan neighborhood just west of central Hong Kong seeped into packed residential buildings, hotels and air-conditioning systems.

The neighborhood is home to Beijing’s liaison office, a target of previous protests.

Ed Vinales, a 38-year-old expatriate from Britain, said he was going down the steps of his apartment building on his way to dinner when he caught a whiff of the gas. He said he was more than 100 yards from the clashes.

“I was still within the iron gates, by the security guard, and I could feel the tear gas already,” he said. He filmed a quick video to post to Instagram and then “literally ran.” 

“It was very invasive and very surprising,” he added. 

Others reported the gas seeping into their apartments and fast-food restaurants. 

Police said they had “liaised with the management of the residential buildings in the neighborhood, and reminded them about the imminent protests and police’s prospective actions.” They have also warned people to stay indoors.

Protesters armed themselves with iron poles and threw bricks at police. Riot police were seen making arrests. A police spokeswoman did not immediately provide details. Four people were injured as of 10 p.m. local time, according to Hong Kong’s Hospital Authority.

Hong Kong’s escalating tensions were sparked by an unpopular extradition bill that would allow fugitives to be sent to China, and it has deepened as the government has declined to fully withdraw the proposal.

In recent days, protesters have grown infuriated with Hong Kong’s police and government, which protesters say are doing the bidding of Beijing rather than protecting citizens and their rights.

On Saturday, tens of thousands of demonstrators gathered in Yuen Long to voice their anger at violent mobs who a weekend earlier had attacked protesters returning to that area after a huge march. The demonstrations turned into a standoff with police at the railway station where the mobs had attacked the weekend before, leaving trails of blood in the suburban area for the second time in six days.

Police arrested 11 men, ranging in age from 18 to 68, in connection with Saturday’s protest “for offences including unlawful assembly, possession of offensive weapon, assaulting police officer and assault,” they said in a statement.

This weekend’s protests, unlike previous marches, were not sanctioned by authorities. Ventus Lau, one of the organizers of Sunday’s rally, said the lack of authorization served to worsen the clashes with police.

“No matter if the march is permitted or not, conflicts may still happen,” he said. “But the letter of objection [from the police] would only make things worse because everything would be less predictable.”

Police throughout the evening urged demonstrators to leave the scene. But after weeks of sustained and highly coordinated protests, the mostly young demonstrators have become more sophisticated in protecting themselves against potential police action, covering their faces with respirators and wrapping exposed skin in cling film to minimize the sting of tear gas.

Protesters have also become bolder and more provocative against riot police, ripping up bricks from the sidewalks, fashioning shields from metal and wood and setting fires to keep police back. At one point Sunday, a protester wearing a full-face gas mask picked up a canister of tear gas that landed at his feet back and threw it back at officers.

Police said protesters had “set ablaze a cart with miscellaneous objects and pushed them toward the police cordon line.”

Hong Kong leader Carrie Lam, one of the main targets of the protesters’ wrath, has largely stayed out of public view as tensions have escalated. But she resurfaced Sunday morning to speak at the closing ceremony of a youth military camp hosted by Chinese troops stationed in Hong Kong. 

Lam, appearing with Wang Zhimin, China’s top representative in Hong Kong, praised the “one country, two systems” framework and urged participants to pass on what they learned at camp to their peers.

Protesters’ demands have swelled beyond the extradition bill that sparked the crisis and now include the long-fought demand that Hong Kong directly elect its leader, who currently must be prescreened by Beijing and is selected by a small committee.

“Liberate Hong Kong!” they chanted Sunday. “This is the revolution of our times!”

Timothy McLaughlin contributed to this report.