Shortly after opening, the benchmark Hang Seng Index was 648.68 points lower at 19,217.95.

Stock markets in Tokyo and Australia were also sharply down after US and European markets tumbled last week.

With G7 finance chiefs meeting in France on Friday not proposing much in the way of concerted intervention, new rumours of an imminent Greek default added to the sell-offs, even as the rumours were quickly denied by Athens.

And markets got a shock when Juergen Stark, the European Central Bank's chief economist, announced his resignation "for personal reasons" well before the end of his term of office in May 2014.

Coming in the middle of an increasingly vocal spat between the ECB and the continent's most powerful economy, Germany, Stark's exit heightened worries that the European Union will not come together to prevent a Greek default or weakening of the eurozone itself.