More than 200 flights were cancelled on Monday, Aug. 5 at Hong Kong International Airport (HKIA), according to the South China Morning Post.

Air traffic controllers have collectively called in sick, forcing authorities to close one of two runways until Tuesday, Aug. 6.

This is part of a citywide strike that is estimated to involve around 500,000 Hongkongers from more than 20 business sectors.

Around half the flights are affected

On Sunday, Aug. 4, more than 20 air traffic controllers took sick leave, accounting for one-third of the officers on duty.

The number of flights that can take off per hour has been halved from 68 per hour to 34 per hour, and flights around Asia are the most affected.

The majority of the affected flights are from Cathay Pacific, Hong Kong's national carrier.

The airline has advised against non-essential travel, and offered waivers for affected passengers to re-book or re-route their travel.

Singapore Airlines flights not affected

Other airlines affected include Cathay Dragon, Cathay Pacific's regional subsidiary, and Hong Kong Airlines.

Flights on Singapore Airlines and Scoot to Hong Kong were not affected.

Several flights from Hong Kong to Singapore were cancelled, according to the HKIA website.

On the other hand, with the exception of Cathay Pacific Flight 636, flights from Singapore to Hong Kong were not affected.

Pilots and flight attendants also on strike

Air traffic controllers were not the only ones who called in sick.

Flight attendants, pilots and other airline staff have also called in sick, as part of the ongoing strike.

The Airport Express train service, which connects the airport to the city, has also been suspended.

Seven simultaneous rallies

Hong Kong Chief Executive Carrie Lam strongly condemned the strikes and the escalating violence, in a press conference held on Aug. 5.

However, the citywide strike is unlikely to lose momentum.

Protesters began gathering at several MTR stations island-wide on Monday morning.

On Monday afternoon, rallies are expected to be held in seven locations -- Admiralty, Mong Kok, Wong Tai Sin, Tai Po, Sha Tin, Tseun Wan and Hong Kong International Airport.

Top image from Hong Kong International Airport.