Thousands of university students in Hong Kong held rallies on Monday, starting a two-week boycott of university classes. Many high school pupils also skipped their classes and joined demonstrations on the first day of the new academic year following the summer break. 

Protesters earlier in the day kept train doors from closing at stations, causing major delays on the local train network during the morning commute.

The demonstrations continue months of activism by a leaderless movement that has drawn millions to the streets to protest restrictions on freedoms and interference in Hong Kong's affairs by China. Students have been at the core of the movement. 

Protesters also called for a general strike on Monday.  A citywide stoppage in August resulted in transport chaos.

Hong Kong's flagship carrier, Cathay Pacific, has threatened to fire staff members who join Monday's strike. The airline has already fired at least four staff members, including two pilots, for supporting the protests.

Read more: EU considers Hong Kong developments 'extremely worrying'

As a Special Administrative Region of China, Hong Kong operates under a "one country, two systems" framework, and under a set of laws agreed to with Beijing, Hong Kong's citizens enjoy civil rights denied to people who live on the mainland, such as freedom of assembly and freedom of speech. 

China committed to the system as a condition of Britain's handing over the colony in 1997. The Chinese government's erosion of those rights has driven the protest movement.

Read more: Hong Kong critical to China but uncertainty reigns

'Horrifying' police violence

On Sunday, officials canceled flights as protesters blocked routes to the airport and police fought demonstrators who attempted to converge on the terminal. "We plan to disrupt activity at the airport to draw attention to what the government and the police are doing to us," one 20-year-old protester said.

Protesters also demonstrated in the city center on Saturday, occasionally setting fires and throwing petrol bombs at riot police, who attempted to prevent them from assembling. Officers struck back with tear gas, baton charges and water cannon laced with chemical dye. Video footage captured by local media showed police charge and beat a crowd cowering inside a train carriage, with Amnesty International calling the actions of the officers "horrifying."

The protests started in June, in opposition to government plans to allow extradition to the mainland of people wanted by China's government, but the movement has widened to include broader demands. These include formally scrapping the extradition bill and an independent inquiry into the violence used by police to put down the protests. The demonstrators also want direct elections for the city-state's leader and all its lawmakers.

kw,mkg/cmk (Reuters, AFP)

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