Hong Kong's major civil aviation labor union voiced strong objections to a mass sit-in at Hong Kong International Airport — warning it is turning the city's key transport hub into a political bargaining chip to blackmail the government.

The Staff and Workers Union of Hong Kong Civil Airlines issued a statement as hundreds of protesters started the sit-in against the now-suspended extradition bill Friday afternoon.

The union spoke on behalf of seven affiliated unions of different occupations. "HKIA is the economic lifeline of the city. Occupying the airport will only damage the city's economy and the livelihoods of airport staff, as it has been recently proved that these protests will end in violent clashes."

The union appealed to fellow members and Hong Kong citizens not to participate in the rally or any campaign intended to undermine airport operations for political purposes.

A citywide strike on Monday led to the cancellation of some 250 flights. On July 26, thousands of black-clad protesters occupied the airport's arrival hall to stage a similar sit-in protest, which caused great inconvenience to many passengers.

The sit-in protest in the airport arrival hall is said to continue for the weekend. The city's Airport Authority has strengthened its safety checks and only departing passengers with travel documents will be allowed to enter the terminal.

In view of these and other previous transgressions, the union called on the government and police to restore peace and order in the city and at the airport for the interests of Hong Kong and its people.

Friday's sit-in has already brought disruptions to travelers. An Australian visitor, who gave his name only as Hugh, said protesters' obstruction of public transportation was annoying during his three-day stay in the SAR. He is concerned about the current situation, and hopes the protests will remain peaceful.

Sandy Kaur, an Indian who has been living in Hong Kong for 12 years, said the rally would make a very bad impression on visitors to Hong Kong and damage the city's international image.

She said such actions were not the right way for people to express their opinions. It was very annoying that there were protests and chaos all over Hong Kong — even at the airport.

A man in the IT industry surnamed Leung, who came back from a business trip, was hit on the face by a man wearing sunglasses and a black shirt on his way to the taxi stand, for criticizing protesters who kept shouting slogans and pestering arriving travelers.

A peaceful rally at the airport was fine, but shouting slogans such as "free Hong Kong" and waving national flags of the United States and the United Kingdom were shameful to Hong Kong, he said.