Thousands of pro-democracy protesters in Hong Kong defied fresh volleys of tear gas early on Monday as authorities tried to quell spreading unrest.
Overnight, riot police advanced on crowds who ignored official warnings that the demonstrations were illegal.
Protesters are angry at Chinese government plans to vet candidates in Hong Kong's 2017 elections.
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.
"Police will strive to maintain social order, including ensuring smooth traffic and ensuring public safety."
Thousands of protesters remained camped out around the government complex overnight, despite appeals for them to go home. Many have erected barricades.
Please turn on JavaScript. Media requires JavaScript to play.
The BBC's Carrie Gracie reports from the heart of the protest
And in a sign that the unrest was spreading, fresh demonstrations sprang up in other areas.
About 3,000 people blocked a major road across the bay in Mongkok while a crowd of about 1,000 faced a line of riot police in the busy shopping district of Causeway Bay, east of central Hong Kong.
With the Monday rush-hour due to start, police issued a statement urging protesters to "stay calm, stop charging police cordon lines and occupying the main roads".
The Hong Kong Education Bureau also announced on Monday that schools in Wan Chai as well as the Central and Western districts would be closed.
Police have so far used batons, tear gas and pepper spray against demonstrators, with varying degrees of success.
However, protest leaders have called on demonstrators to retreat if rubber bullets are used.
"This is a matter of life or death. If their lives are threatened they should retreat and save their lives," said professor Chan Kin-man, a co-founder of the Occupy Central group.
The broader Occupy Central protest movement threw its weight behind student-led protests on Sunday, bringing forward a mass civil disobedience campaign due to start on Wednesday.
Police said they had arrested 78 people on Sunday on charges of "forcible entry into government premises, unlawful assembly, disorderly conduct in public place, assaulting public officers and obstructing police officers in due execution of duties".
It followed the arrest of more than 70 people in Saturday's protests, including prominent student activist leader Joshua Wong, who has since been released.
Please turn on JavaScript. Media requires JavaScript to play.
Chief Executive CY Leung said the government was "resolute in opposing the unlawful occupation" by protesters
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.
A spokesman for China's Hong Kong and Macau affairs office said that Beijing "firmly opposes all illegal activities that could undermine rule of law and jeopardise 'social tranquillity' and it offers its strong backing" to the Hong Kong government, Xinhua news agency reports.
However, 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