As unrest continues in Hong Kong for its 14th week, police have thwarted demonstrators' attempts to choke the city's international airport by screening train passengers and deploying tear gas for a second consecutive night.

Key points: Hong Kong's airport has become a major battleground between protesters and police

Hong Kong's airport has become a major battleground between protesters and police The city's metro selectively closed subway stations to hinder protesters from gathering

The city's metro selectively closed subway stations to hinder protesters from gathering Unrest has led credit agency Fitch Ratings to downgrade Hong Kong from AA+ to AA

Police checked for tickets and passports to allow only airline passengers through to the airport to avoid the chaos of last weekend, when activists blocked approach roads, threw debris on to train tracks and trashed the nearby subway station in Tung Chung.

Protesters also occupied the arrivals hall last month, halting and delaying flights amid a series of clashes with police.

Some metro stations, such as Prince Edward Station, were closed to stifle the mobility of protesters across Hong Kong. ( AP: Kin Cheung )

More than three months of protests have at times paralysed parts of Hong Kong as running street battles between protesters and police have disrupted the day-to-day processes of the city.

On Friday night hundreds of demonstrators, many masked and dressed in black, attacked MTR metro stations in the Mong Kok district.

Activists, angry that the MTR closed stations to stop protesters from gathering and demanding CCTV footage of the beatings, tore down signs, broke turnstiles, set fires on the street and drew graffiti on walls.

Protesters gathered again after nightfall on Saturday to be dispersed by tear gas followed by running clashes with police who fired pepper spray. Several arrests were made.

"We didn't have the numbers," said one male masked protester who fled from Prince Edward station, pursued by riot police.

"We'll be back."

There were also clashes at the rail station in Sha Tin, north of the landmark Lion Rock, which separates downtown Kowloon from the New Territories bordering mainland China.

Lam's concessions didn't prevent more protests

Some protesters attacked metro stations in relation for their strategic closures. ( AP: Kin Cheung )

The protests mark the biggest challenge to Chinese President Xi Jinping's rule within Hong Kong since he came to power in 2012.

Despite Hong Kong chief executive Carrie Lam announcing concessions to protesters this week — including formally withdrawing a hugely unpopular extradition bill — the move did not stop further protests from taking place.

The bill would have allowed extraditions of people to jurisdictions that did not have formal extradition treaties with Hong Kong, which includes mainland China — a decision that prompted fears of the stifling of dissent from within Hong Kong.

The withdrawal of the bill answered one of five formal demands made by protesters, which also include universal suffrage; the release of all arrested protesters; the de-classification of a June protest as a 'riot'; and the formation of an independent commission of inquiry into police behaviour.

Many said the concessions were too little, too late.

Ms Lam also refused to institute an independent commission of inquiry — instead she said Hong Kong's independent police complaints council was credible enough.

Hong Kong's credit rating downgraded a notch

A memorial for protesters who were injured in Mong Kok was set up around a closed metro station entrance. ( AP: Kin Cheung )

Hong Kong is facing its first recession in a decade, which business leaders say is due to the unrest.

Credit rating agency Fitch Ratings downgraded Hong Kong's long-term foreign currency issuer default rating to "AA" from "AA+" on Friday.

The demonstrations, which began in June, have grown into a broader existential debate about Hong Kong under Beijing's rule.

The former British colony was transferred back to China in 1997, which involved the "one country, two systems" framework, which guaranteed the territory British capitalism, common law, and freedoms not found on the mainland.

Police deployed tear gas on protesters for the second consecutive night on Saturday. ( Reuters: Kai Pfaffenbach )

While this is due to last until 2047, many Hong Kong residents fear Beijing is already eroding that autonomy.

While China has denied accusations of meddling in the semi-autonomous territory's affairs, others have pointed to the build-up of the People's Liberation Army's soldiers and military vehicles in the border city of Shenzen as proof of Beijing's potential to intervene.

So far, the mainland has denounced the protests, and has accused the United States and Britain of fomenting unrest, and warned of the damage to the economy.

German chancellor Angela Merkel, who is currently on a state visit to China, told reporters that she had pressed Beijing for a peaceful resolution to the crisis.

"I have advocated that conflicts be resolved without violence and that anything else would be a catastrophe from my point of view," Ms Merkel said.

"This is important."

Riot police were screening users of train services to the airport. ( AP: Vincent Yu )

ABC/Reuters