Thousands of students in Hong Kong have boycotted university and secondary school classes to take part in peaceful rallies.

The boycott follows a weekend of violence that saw protesters burn barricades and throw petrol bombs, while police retaliated with water cannon, tear gas and batons.

University and secondary school students gathered at the Chinese University campus for a demonstration, with many seen singing, chanting and forming human chains.

China has accused western countries of egging on the protests, and China's Foreign Ministry spokesperson, Geng Shuang, reiterated Beijing's support for Hong Kong leader Carrie Lam.

"China's central government supports chief executive Carrie Lam's leadership ... and supports Hong Kong's police tackling the violence and chaos in accordance with the law to restore order."

The Secretary for Education, Kevin Yeung, told reporters that schools were no place for "raising political demands" or trying to pressure the government.

"We would like to keep schools as calm, peaceful and orderly places for students to study," he said.

The unrest began over a now-suspended extradition bill that would have allowed people in the city to be sent to China for trial in courts controlled by the party. 

The turmoil has evolved into calls for democracy. China is eager to quell the unrest before the 70th anniversary of the founding of the People's Republic of China on 1 October.

With Hong Kong facing its first recession in a decade, China has also warned of the damage the protests are causing to the economy.