Hong Kong’s Airport Authority cancelled all flights not yet checked in by Monday afternoon, the agency said, as anti-government protesters peacefully demonstrated at the airport for a fourth day.

“Other than departure flights that have completed the check- in process and the arrival flights already heading to Hong Kong, all other flights have been cancelled for the rest of today,” the authority said in a statement.

Traffic on roads to the airport was very congested and carpark spaces were full, the authority said.

The increasingly violent protests have plunged Chinese-ruled Hong Kong into its most serious crisis in decades and presented a serious challenge to Beijing.

Last week, Canada increased its travel advisory for Hong Kong due to the ongoing protests, advising travellers to exercise “a high degree of caution.” The move came after the U.S. and Australia made similar updates to their travel alerts; Ireland, Britain and Japan have also issued travel advisories to their citizens.