Some votes were still being counted Monday morning, but Hong Kong media tallied that the pro-democracy camp had easily won a majority in the vote for 452 district council seats.
The result could force the central government in Beijing to rethink how to handle the unrest, which is now in its sixth month. The district councils have little power, but the vote became a referendum on public support for the protests.
The pro-democracy camp hailed its strong gains in the normally low-key race as a "victory" for the Hong Kong people. Candidates said Lam must heed protesters' demands including free elections for the city's leader and legislature, and an independent probe into alleged police brutality.
"We won a small battle today but it shows that Hong Kong people have a chance to win the war. We will fight on," said Henry Sin Ho-fai, a pro-democracy candidate who won.
A record 71% of the city's 4.1 million registered voters cast ballots Sunday, well exceeding the 47% turnout in the district council elections four years ago. The large number of votes was slowing down the counting.
Hong Kong's largest pro-Beijing political party suffered the biggest setback, with at least 155 of its 182 candidates defeated, according to media tallies.
Among the losing incumbents was controversial lawmaker Junius Ho, who was stabbed with a knife while campaigning this month.
The winners included former student leaders and a candidate who replaced prominent activist Joshua Wong, the only person barred from running in the election. Pro-democracy rally organizer Jimmy Sham, who was beaten by hammer-wielding assailants last month, also triumphed, as did a pro-democracy lawmaker who had part of his ear bitten off by an assailant.
The demonstrations have turned increasingly violent. Protesters have smashed storefronts of businesses seen as sympathetic to China, torched toll booths, shut down a major tunnel and engaged in pitched battles with police, countering tear gas volleys and water cannons with torrents of gasoline bombs.
More than 5,000 people have been arrested in the unrest that contributed to Hong Kong's first recession in a decade.
The vote is the only fully democratic one in Hong Kong. Members of the legislature are chosen partly by popular vote and partly by interest groups representing different sectors of society, and the city's leader is picked by a 1,200-member body that is dominated by supporters of the central government in Beijing.
Dixon Sing, a political science lecturer with the Hong Kong University of Science and Technology, said the democrats' victory was "tantamount to a rejection of the hard-line policy of Beijing and the Hong Kong government."
A win would bolster the democrats' influence and give them 117 seats in the panel that elects the city's leader, but Beijing isn't likely to soften its stance or make any concessions to the protesters, he said.
END/AP/UNB/TZ