Hong Kong leader Carrie Lam has allegedly scrapped a meeting with US senator Ted Cruz, the highest-profile American politician to visit the city since anti-government protests broke out more than four months ago.

Key points: Senator Cruz says the cancellation is a "sign of weakness"

Senator Cruz says the cancellation is a "sign of weakness" Petrol bombs were reportedly thrown in a metro station

Petrol bombs were reportedly thrown in a metro station Elderly protesters have begun a 48-hour sit-in at a police station

Ms Lam's office had requested the afternoon meeting be completely confidential and that Mr Cruz refrain from speaking with the media about it, Mr Cruz told journalists in Hong Kong.

"She seems to misunderstand how free speech operates, and also how freedom of the press operates," said Mr Cruz, a Republican senator from Texas and a vocal critic of China who was stopping in Hong Kong for two days as part of a regional tour.

"Ms Lam's cancelling the meeting is not a sign of strength. It's a sign of weakness," he said.

Ted Cruz said he wore black to show support with pro-democracy protesters in Hong Kong. (Reuters: John Ruwitch)

"It's a sign of fear of the protesters in the streets of Hong Kong."

Responding to a request for details about the scheduled meeting, Ms Lam's office said in an email: "The Chief Executive did not meet with the said US senator."

Molotov cocktails thrown in Hong Kong metro

Demonstrators meanwhile took to the streets of Hong Kong again on Saturday. Petrol bombs were thrown inside Kowloon Tong metro station causing serious damage, police said.

Hundreds of protesters, many young and wearing face masks, were marching in Kowloon at the time and were headed to a district near the Kowloon Tong station.

Protesters marching peacefully hit the rain-slickened streets of Hong Kong again in multiple locations on Saturday. (AP: Felipe Dana)

About a dozen riot police took to the streets in Kowloon's Tsim Sha Tsui district, normally a haven for local and international shoppers, behind the marchers shortly after news of the petrol bomb attack.

Hong Kong's metro has borne the brunt of protests, with stations torched and trashed, and only returned to normal operations on Friday after being completely shut down. The metro normally carries around 5 million people a day.

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A small group calling itself the "Silver-Haired Marchers" began a 48-hour sit-in at police headquarters on Saturday, describing themselves as "old but not obsolete".

"Whilst we may not be able to fight alongside the young protesters in the frontline against an unjust government, escalating police violence and indiscriminate arrests, we take it to heart to uphold the core values of Hong Kong and defend the future of our younger generations," it said in a statement.

Colonial-era emergency laws were introduced a week ago banning face masks at public rallies, sparking some of the worst violence since the protests started. Protesters use masks to shield their identities.

However, hundreds of people, including school children and office workers, have since defied the ban and worn face masks. A group of protesters plan a "face mask party" on Saturday night.

Protests that started in June over a now-shelved extradition bill have since snowballed into an anti-China campaign. (AP: Kin Cheung)

Millions 'yearning to live free', says Ted Cruz

"I stand with the people of Hong Kong calling on the government of China to honour the promises it made to the world when it promised to maintain political freedom in Hong Kong," said Mr Cruz, who wore black to show support for the black-clad protest movement.

Asked if he condemned violence that has flared during the protests, Mr Cruz said he advocated non-violent protest with the protesters and democracy activists he had met.

Hong Kong's police, once praised as "Asia's finest", are also facing a crisis of confidence amid the worsening political tensions.

Protesters accuse them of using excessive force, a charge police deny, and two protesters have been shot and wounded during skirmishes with police.

Mr Cruz said he believed Chinese President Xi Jinping was "terrified of millions of people in Hong Kong, but even more than that, millions of people in China yearning to live free".

Many Hongkongers have defied a ban on face masks. (AP: Felipe Dana)

Those fears were "magnified on the world stage" by China's response to a tweet by a US basketball executive supporting Hong Kong's protesters, he said.

China has accused the West of stirring up anti-Beijing sentiment in Hong Kong, and Chinese state media characterised the Houston Rockets general manager's tweet as the latest example of meddling in China's affairs.

There is "overwhelming bipartisan support" in the US Congress for the people of Hong Kong, Mr Cruz said, adding he was pressing for the Senate to take up and pass the Hong Kong Human Rights Act quickly.

Reuters