Local shares snapped a six-day winning streak on Monday, tumbling along with other regional stocks, as the city grappled with a new wave of infections and further tightened social distancing measures.
The Hang Seng Index had fluctuated in a narrow range before the losses widened in the afternoon.
The benchmark closed down 553 points or more than 2 percent, at 26,341, on turnover of HK$240.7 billion
Still, the index gained 9.3 percent for the month.
Heavyweight Tencent slipped 3.4 percent and Alibaba lost more than 3 percent on Monday.
But Xiaomi bucked the trend and jumped 1.7 percent.
Mainland oil firms were battered after reports said CNOOC would be added to a blacklist that would bar US investors from buying its shares. The oil giant sank close to 14 percent to become the worst performing blue-chip. Sinopec and PetroChina each slid more than 6 percent.
Markets across the border also finished lower despite positive data on mainland manufacturing activity, with sentiment hampered by concerns over friction between China and the United States. The Shanghai Composite Index retreated half a percent, while the blue-chip CSI300 Index fell 0.4 percent. The Shenzhen Composite Index edged down 0.2 percent.
Across the region, Japan's benchmark Nikkei 225 dipped 0.8 percent on Monday, but it still managed to log its biggest monthly gain since 1994. Seoul's Kospi eased 1.6 percent. Taiwan fell 1 percent. Singapore and Australia each gave up more than 1 percent.