Several thousand protesters thronged into Hong Kong airport on Friday without reporting with the police in advance, putting pressure on the airport, one of the world's busiest, and stoking concerns of passengers about delay or cancellation of their flights.

The black-clad demonstrators, who mostly wore masks, started to sit on the ground of the arrival hall of the Hong Kong International Airport (HKIA) at noon, shouting out anti-government slogans.

Arrivals were left narrow paths lined by demonstrators and had to squeeze through the crowds carrying their suitcases and bags.

The rally, expected to last for three days, has not been reported with the police according to rules on public gatherings.

Several civil aviation workers groups on Friday issued statements to oppose the rally, saying it is illegal and will damage Hong Kong's image and the economy. They also called on residents in Hong Kong not to participate in the rally.

To deal with the protesters, the airport authority sent out personnel to keep order and strengthened entry management.

Only departure passengers taking tickets and valid passports and airport staff will be allowed to enter the check-in aisles at the terminal one, according to a statement of the airport.

HKIA, connecting to over 220 destinations worldwide, handled 74.7 million passengers and 427,700 flights last year. It was ranked as the world's top-10 airport for shopping, leisure amenities, dining, airport transit, hotels and airport security, cleanliness and staff services at the annual Skytrax World Airport awards for 2019.

Several protesters waved US and British flags. A woman in white questioned the behavior in Cantonese and called them "traitors."

"You are Chinese. You are Hong Kong residents. You should love the place and mustn't mess up the place," said the woman.

A tourist from Malaysia, who asked to be quoted as Christina, said she read about violent incidents in Hong Kong before arriving here with companions but could not change the schedule as the flight tickets were booked in advance.

"We will try to stay clear of crowds," she said.

Sarah Pirker heads for Austria after a four-night trip in Hong Kong.

"It is fortunate that my flight has so far not been affected," she said.

New Zealand tourist Mark Dix and his friend were waiting for the transferring flight to New Zealand and had to lean against a pillar as protesters occupy most of the space, with several handouts shoved to their hands.

"Our travel plan has not been affected but I am still a little bit uncomfortable reading stories about violent incidents, in which protesters stepped across the line," he said. "I hope the unrest will end as soon as possible."