HONG KONG — Black-clad protesters hurled gasoline bombs at government offices in central Hong Kong on Sunday, as a day that began with a peaceful march by tens of thousands of pro-democracy demonstrators descended into clouds of tear gas deployed by the police and ugly brawls between civilians.

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.