Media playback is unsupported on your device Media caption China editor Carrie Gracie asks workers in Hong Kong's financial industry if they believe the protests will harm the economy

Tens of thousands of pro-democracy protesters are blocking Hong Kong's streets, shutting down its business hub and ignoring appeals to leave.

The demonstrations have spread to other areas including a shopping district and a residential area.

Riot police withdrew on Monday after overnight clashes in which they used batons and fired volleys of tear gas to try to disperse the crowds.

China has warned other countries not to support the "illegal rallies".

Its foreign ministry said it opposed any interference in China's internal affairs.

Meanwhile the British government called for the right to protest to be protected.

"It is important for Hong Kong to preserve these rights and for Hong Kong people to exercise them within the law," the UK foreign office said in a statement.

Protesters - a mix of students and supporters of the Occupy Central pro-democracy movement - are angry at Beijing's plans to vet candidates for Hong Kong's 2017 leadership elections.

They want a free choice of candidates when they cast their ballots for the chief executive - something Beijing says is out of the question.

Media playback is unsupported on your device Media caption "Tired yet defiant": The BBC's Laura Westbrook reports on the protesters refusing to leave

Sunday saw angry scenes and dozens of arrests on Hong Kong's streets as tens of thousands of protesters faced riot police in the heart of the city.

In a news conference on Monday, Cheung Tak-keung, assistant commissioner of police for operations, said police had used the "bare minimum force".

He said 41 people had been injured in the past three days, including 12 police officers.

The Hong Kong government urged protesters to stay calm and leave peacefully but crowds remained camped out around the government complex on Monday night.

About 3,000 people have also blocked a major road across the bay in Mongkok, on the Kowloon peninsula, while a crowd of about 1,000 faced police in the busy shopping district of Causeway Bay, east of central Hong Kong.

Image copyright AFP Image caption Protesters remained on the streets on Monday evening, with many camped outside government buildings

Image copyright Reuters Image caption About 3,000 protesters blocked a major road across the bay in Mongkok, on the Kowloon peninsula

Image copyright AP Image caption Some protesters covered their faces with plastic wrap and goggles as an anti-tear-gas measure

Image copyright Getty Images Image caption Earlier, police clashed with protesters attempting to block roads in central Hong Kong

Schools in the Wan Chai, Central and Western districts were closed on Monday and will remain shut on Tuesday, according to the Hong Kong Education Bureau.

The city remains heavily disrupted, with several major thoroughfares blocked.

One man said protesters were growing more confident. "Police don't have enough officers to close down the districts where there are protests," Ivan Yeung, 27, told AFP news agency.

Hong Kong's chief executive reassured the public that rumours the Chinese army might intervene were untrue.

"I hope the public will keep calm. Don't be misled by the rumours," CY Leung said.

China, which stations a garrison of the People's Liberation Army (PLA) in Hong Kong, said it was confident the city's administration could handle the protests.

In other developments:

Police said they used tear gas 87 times in clashes with protesters on Sunday

More than 200 bus routes have been cancelled or diverted; some subway exits in protest areas have been blocked

Several banks have suspended operations in affected areas

Police said they arrested 78 people on Sunday, after 70 arrests on Saturday.

In the Taiwanese capital, Taipei, protesters gathered outside the Hong Kong cultural office in a show of support

President Ma Ying-jeou said Taiwan was closely watching the situation in Hong Kong

In mainland China, reports say Instagram has been blocked, it is thought due to the protests

Media playback is unsupported on your device Media caption Chief Executive CY Leung said the government was "resolute in opposing the unlawful occupation" by protesters

'Sorrow over chaos'

Tensions escalated on Sunday when the broader Occupy Central protest movement threw its weight behind student-led protests, bringing forward a mass civil disobedience campaign due to start on Wednesday.

In a statement on Monday, the movement called on Mr Leung to step down, saying "only this will make it possible to re-launch the political reform process and create a space in which the crisis can be defused".

Celia Hatton: The view from Beijing

China's leaders must be sitting uncomfortably in Beijing.

As long as the protests continue, there is a chance they will spread to the mainland, where many are unhappy with one-party rule. The Chinese government is taking clear steps to limit information about events in Hong Kong by censoring internet search terms and forums discussions.

But if the protesters hold their ground, how far will Beijing allow events to spiral before getting directly involved?

The sight of Chinese troops confronting Hong Kong protesters, particularly students, would be a disaster for Beijing, leading to an international outcry. Beijing could revisit the dark days following its violent response to 1989's Tiananmen protests.

So, for now, Chinese leaders face an unusual set of political constraints. The Communist Party is unwilling to cede political control to the people of Hong Kong by refusing to allow direct elections in 2017. As a result, the party is putting its faith in the abilities of the Hong Kong police to deal with the fall-out from that decision.

A spokesman for China's Hong Kong and Macau affairs office said that Beijing firmly opposed "all illegal activities that could undermine rule of law and jeopardise 'social tranquillity,'" Xinhua news agency reports.

Mainland newspapers have blamed "radical opposition forces" for stirring up trouble.

Analysts say Communist Party leaders in Beijing are worried that calls for democracy could spread to cities on the mainland.

Britain handed Hong Kong back to China in 1997 under a "one country, two systems" formula that guarantees liberties not seen on the mainland, including freedom of speech and the right to protest.

Hong Kong democracy timeline

1997: Hong Kong, a former British colony, is handed back to China under an 1984 agreement giving it "a high degree of autonomy, except in foreign and defence affairs" for 50 years

2004: China rules that its approval must be sought for changes to Hong Kong's election laws

June-July 2014: Pro-democracy activists hold an unofficial referendum on political reform and a large rally. This is followed by protests by pro-Beijing activists

31 August 2014: China says it will allow direct elections in 2017, but voters will only be able to choose from a list of pre-approved candidates. Activists stage protests

22 September 2014: Student groups launch a week-long boycott of classes in protest

2017: Direct elections for chief executive due to take place

2047: Expiry of current agreements

Q&A: Hong Kong's democracy controversy