Riot police clash with demonstrators during a protest in Hong Kong. (Reuters)

Hong Kong police used tear gas to break up anti-government protests in a gritty industrial suburb on Saturday after some activists threw petrol bombs and bricks, as China freed a British consulate worker whose detention helped fuel tension.

Four MTR subway stations were closed around Kwun Tong, a densely populated area of the Chinese-ruled city on the east of the Kowloon peninsula, but thousands packed the streets anyway, most carrying umbrellas against the sun.

Police used tear gas after some protesters threw Molotov cocktails and bricks and others tore up "smart" lamp posts equipped with surveillance cameras. Others had set up roadblocks with bamboo scaffolding.

It was the first use of tear gas in 10 days after a series of mostly peaceful demonstrations in the former British colony.

There were no immediate reports of injuries.

"Give me democracy or give me death," was spray-painted on a wall, an illustration how the demands of the protesters have expanded from the withdrawal of a bill that would have allowed extraditions to China.

The government said in a statement the protesters "posed a serious threat to the safety of everyone" at the scene.

"After repeated warnings to the protesters went futile, police officers have deployed tear gas and minimum force to disperse protesters," it said.

The airport and the roads and railways leading to it were operating normally despite plans by protesters to implement a "stress test" of transport links after weeks of unrest.

The airport was forced to close for part of last week after protesters thronged the main terminal for several days, grounding around 1,000 flights and occasionally clashing with police.

The wider calls for democracy have plunged the city into an unprecedented crisis posing a direct challenge for Communist Party leaders in Beijing.

Demonstrators say they are fighting the erosion of the "one country, two systems" arrangement that enshrines a high degree of autonomy for Hong Kong since it was handed back from British to Chinese rule in 1997.

Hundreds took part in an "anti-fake news" protest, with people waving the bauhinia Hong Kong flag outside government-funded broadcaster RTHK's headquarters.

"RTHK has become more biased in recent years," businesswoman Michelle Tse said. "Almost 90% of its programnes have been badmouthing China, saying the country has no democracy or freedom."

A spokeswoman for the station said RTHK has taken note of the opinions raised, it said on its Facebook page. "She said staff follow the station's charter and producer guidelines, and are obliged to deliver objective programming."

CONSULATE WORKER FREED

British consulate staffer Simon Cheng was detained for 15 days for violating public security management regulations, police in Shenzhen, across the border from Hong Kong, said on their Twitter-like Weibo account.

Police said Cheng was released as scheduled on Saturday and that his legal rights and interests had been observed. They also said Cheng had confessed to accusations against him, a commonly used comment by Chinese police, even though Cheng was not given a chance to defend himself in court.

Cheng had now returned to Hong Kong, his family said on his Facebook page.

No details were given of his detention, with the Facebook post asking the "media and friends to give them some time and space, and we will explain more later".

Some protesters in recent days had demanded Cheng be released. Britain said it welcomed the news.

Cathay Pacific Airways, which has become the biggest corporate casualty of the protests after China demanded it suspend staff involved in the demonstrations, protested against a planned rally by the Hong Kong Confederation of Trade Unions around the airport's "Cathay City" HQ on Monday.

"We have also reiterated to our employees that there is a zero-tolerance approach to any support for or participation in illegal protests, violent activities or overly radical behaviour," it said in a statement.

There is no sign of a let-up almost three months after the anti-government demonstrations began. On Friday night, thousands of chanting protesters formed human chains around the city in a peaceful protest dubbed the "Hong Kong Way".

Authorities have so far refused to meet any of the protesters' five key demands, including calls for an independent inquiry into police brutality, a full withdrawal of the extradition bill, and full democracy.

Organisers are planning a host of protests in the coming weeks including a mass march, a city-wide strike and class boycotts at universities.