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 resign.

They say they will start occupying government buildings if he does not quit by Thursday night.

Overnight, some 3,000 protesters massed outside his office, AFP news agency reports. There is a heavy police presence outside government offices, the South China Morning Post reports.

But the BBC's Juliana Liu in Hong Kong says crowds on Connaught Road in the Central district on Thursday morning were at their lowest ebb since the protest began.

Organisers have attributed this to sheer exhaustion, as the demonstration stretches into its fifth day, she says.

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, state TV said Hong Kong's police should be supported in their attempts to "restore the social order as soon as possible".

An editorial in the Chinese Communist Party's newspaper People's Daily on Thursday called the situation "chaos".

"Hong Kong has for many years enjoyed peace and harmony. It now sees the emergence of this embarrassing 'chaos', and the root cause lies with a few people who are disrespecting the law.... 'chaos' denies Hong Kong residents their prosperity, goes against their wishes, and is not what all Chinese people wish to see or what we can tolerate."

(BBC)