Hong Kong stocks plunged 628.84 points, or 3 percent, to 20,317.3 during Monday's morning session.

Hang Seng Index tumbled amid global fears and sell-off after rating agency Standard & Poor's downgraded the long-term sovereign credit rating for the United States on Friday from AAA to AA+.

The local benchmark index once dropped as low as 3.7 percent during the morning trading, with various sub-indices fell more than 3 percent.