China's Foreign Minister Wang Yi has issued a stern warning against any "illegal" protests in Hong Kong.
Visiting Washington, Mr Wang also warned that the matter was an "internal affair" for China.
His US counterpart, John Kerry, urged Hong Kong to exercise restraint in dealing with the protests.
Earlier, student demonstrators angry at China's vetting of candidates for 2017 elections vowed to step up protests if Chief Executive CY Leung did not quit.
They said that protesters would start occupying government buildings if Mr Leung did not resign by Thursday.