Hong Kong was on Sunday reeling from one of its worst clashes the morning after police fired tear gas and water cannon on pro-democracy protesters. The demonstrators have planned to block travel routes to Hong Kong's international airport.

At least 16 flights were cancelled, theairport's website said, with the departure hall packed with a backlog ofpassengers who had struggled to make it to the terminals.

Earlier, operators of the Airport Express train suspended services after the station was besieged, while black-clad protesters – hiding from CCTV cameras under umbrellas – built barricades at the bus terminus and attempted to stop traffic on the main road leading to the facility. Stranded travelers were forced to abandon their lifts and drag their luggage along the airport road.

Protesters on Sunday threw petrol bombs, while the police fired rounds of tear gas, water cannon and rubber bullets at them. The escalating clashes have plunged the city into its worst political crisis in decades.

Police arrested 40 people inside PrinceEdward metro station on suspicion of obstructing officers, unlawful assemblyand criminal damage, even as three stations were shut on Sunday.

Police said in a statement "A large group of protesters participated in unlawful assembly in various districts since yesterday, despite police's objection and warning," "The level of violence is rapidly escalating and their illegal acts have no regard to the laws of Hong Kong."

Saturday's protests came on the fifthanniversary of a decision by China to curtail democratic reforms and rule outuniversal suffrage in Hong Kong, a former British colony that was returned toChina in 1997.

The protests had initially been organized tooppose a bill that would have allowed extraditions to China. They have nowevolved into a backlash against the city's government and its political mastersin Beijing.

The government has refused to accept any of the protesters' main demands, which include a complete withdrawal of the extradition bill besides an independent inquiry into the use of excessive police force against the demonstrators. They are also demanding the resignation of the city's leader Carrie Lam.

Last month, Beijing had claimed that criminals and agitators were stirring violence, encouraged by foreign powers such as Britain and the United States.