All flights out of Hong Kong airport were cancelled on Monday after thousands of demonstrators occupied one of the world’s busiest travel hubs, as Beijing denounced the protests as "terrorism".

The sudden airport shutdown came as mass demonstrations spilled into a third month, despite rising threats from the authorities.

On Monday, Beijing officials gave their third press conference in as many weeks, a rarity for authorities in normal times.

“Radical protesters have been frequently using extremely dangerous tools to attack the police in recent days, constituting serious criminal acts with sprouts of terrorism emerging,” said Yang Guang, a spokesperson for the Hong Kong and Macau Affairs Office in Beijing, which reports to China’s cabinet.

Violence escalated significantly between protesters and police over the weekend, as officers shot tear gas into underground subway stations for the first time after mass demonstrations began early June. Police have made more than 600 arrests in recent days.