Police hit back at protesters with tear gas, baton charges and water cannon laced with chemical dye.

Hong Kong braced for more disruption Monday with calls for a general strike and university students threatening a boycott of classes after a weekend featuring some of the worst violence in three months of anti-government protests.

The financial hub has been thrown into chaos by a largely leaderless movement that has drawn millions onto the streets to protest against what they see as an erosion of freedoms and increasing interference in their affairs by Beijing.

Authorities have given permission for two rallies on Monday, but it is unclear how well supported they will be, or if they will spill over into violence as most recent gatherings have done.

Universities are due to resume classes Monday after a summer break, but students -- who make up the backbone of the protest movement -- plan a two-week boycott, and a rally in the afternoon.

The protests started in opposition to government plans to allow the extradition to the mainland of people wanted by Beijing, but have widened to include broader demands.

Chief among these are for the extradition bill to be formally scrapped, for the territory's chief executive to quit, and for an independent inquiry into alleged police brutality during the protests.

"This is a leaderless campaign where the like-minded students gather and make class boycott happen," said a boycott organiser, requesting anonymity.

Monday's strike call comes with the city still recovering from a weekend of clashes that saw at least a dozen flights cancelled Sunday after protesters blocked routes to the airport.

On Saturday, hardcore protesters rampaged through the city centre, setting fires and throwing petrol bombs at riot police in defiance of a rally ban.

Police hit back with tear gas, baton charges and water cannon laced with chemical dye.

Video footage captured by local media showed elite police charge and beat a crowd cowering inside a train carriage, with Amnesty International calling their actions "horrifying".

In early August, a city-wide strike -- a rare occurance in a freewheeling finance hub where unions traditionally have little sway -- brought transport chaos to the city.

Hong Kong' flagship carrier Cathay Pacific has warned staff they risk being sacked if they joined Monday's strike after the last walkout was supported by the airline's flight attendant union.

Cathay has already fired at least four staff -- including two pilots -- for supporting the protests.

Hong Kong's reputation as a stable place to do business has been shaken by the ongoing protests.

Visitor arrivals have plummeted, hotel vacancies have soared and retailers have reported huge losses as a result of the protests, but the government has offered little in the way of concessions or suggestions on how to end them.