China's foreign minister has warned the United States and other foreign countries not to meddle in China's "internal affairs", as the country continues to grapple with the escalating protests in Hong Kong.

Wang Yi issued the statement on Wednesday, shortly before meeting US President Barack Obama who told him in a White House meeting that the US was watching the Hong Kong protests closely.

As protesters continued to occupy Hong Kong's business district on Thursday morning, Wang first met his US counterpart John Kerry in Washington before heading to the White House.

Wang told Kerry that "all countries should respect China's sovereignty and this is a basic principle of governing international relations".

"I believe for any country, for any society, no one would allow those illegal acts that violate public order. That's the situation in the United States and that's the same situation in Hong Kong."

Kerry repeated US calls for Chinese authorities to show restraint towards the mass protests challenging the communist government.

Later at the White House, Wang met with Obama and his National Security Adviser Susan Rice, mainly to discuss Obama's visit to China in November, but the meeting was overshadowed by events in Hong Kong.

In a statement, the White House said; "The United States has consistently supported the open system that is essential to Hong Kong's stability and prosperity, universal suffrage, and the aspirations of the Hong Kong people."

Full democracy

The protesters, mostly students, are demanding full democracy and have called on Chief Executive Leung Chun-ying to step down from the top office of the former British colony that reverted to Chinese rule in 1997.

On Thursday morning, thousands of protesters continued to camp out in the main streets of the Chinese autonomous region, and threatened to occupy government buildings if their demands were not met, Al Jazeera's Rob McBride reported from Hong Kong.

The city's streets were calm while police largely kept their distance from the tens of thousands of mostly young people keeping up protests, now nearly a week old.

Al Jazeera's Scott Heidler, who is also reporting from Hong Kong, said some of the protesters had started to occupy the area next to Leung's office as of Thursday morning.

Reuters news agency reported that Leung is willing to let the demonstrations go on for weeks if necessary, while defying calls for him to resign.

The Hong Kong demonstrations were triggered after the Chinese government restricted who can run as the commercial hub's next chief executive, or leader, in elections scheduled for 2017.

The protesters called this "fake democracy" and have two demands - that Leung steps down and that Beijing reverses its decision.

On Wednesday night, one of the student leaders organising the protests threatened to step up the action - including a possible attempt to occupy government offices.

"We need to escalate the movement," said 23-year-old student Jason Chan. "So many people have come out every day and the government hasn't responded to us. If we don't take things to the next level, this movement is pointless."

But many others were reluctant to take any action that could prompt further clashes with the police.

"I think we should keep this a peaceful revolution," said costume designer Janice Pang. "Hong Kong people may not support us if we do something more extreme."