China's Foreign Minister Wang Yi has warned against "illegal" protests in Hong Kong, amid major pro-democracy rallies in the territory.

Mr Wang, in Washington, said the matter was an "internal affair" for China.

US counterpart John Kerry urged Hong Kong to handle the protests - seen as a challenge to Beijing - with restraint.

In Hong Kong, students angry at China's plan to vet candidates for 2017 polls have vowed to step up protests if Chief Executive CY Leung does not quit.

They say protesters will start occupying government buildings if Mr Leung does not resign by Thursday night.

Overnight, some 3,000 protesters massed outside Mr Leung's office in a stand-off with some 200 police, AFP news agency reports. There is a heavy police presence outside government offices, the South China Morning Post reports.

'High hopes'

The protesters include students, supporters of pro-democracy group Occupy Central and others angered by the initial police response to rallies at the weekend, which included using tear gas.

Mr Wang, the most senior Chinese official to speak openly on the matter, said: "Hong Kong affairs are China's internal affairs. All countries should respect China's sovereignty. For any country, for any society, no-one will allow those illegal acts that violate public order."

But he said he believed Hong Kong authorities had "the capability to properly handle the current situation in accordance with the law".

Mr Kerry said the US supported universal suffrage in Hong Kong, adding he hoped local authorities would "exercise restraint and respect for the protesters' right to express their views peacefully".

In China, an editorial in People's Daily warned of "unimaginable consequences" if the protests continued, while state TV said Hong Kong's police should be supported in their attempts to "restore the social order as soon as possible".

Source: BBC