Hong Kong has been rocked by months of sometimes violent unrest. (File)

Hong Kong activists called off protests on Wednesday in remembrance of the September 11, 2001, attacks on the United States and denounced a Chinese state newspaper report that they were planning "massive terror" in the Chinese-ruled city.

Hong Kong has been rocked by months of sometimes violent unrest, prompted by anger over planned legislation to allow extraditions to China, but broadening into calls for democracy and for Communist Party rulers in Beijing to leave the city alone.

"Anti-government fanatics are planning massive terror attacks, including blowing up gas pipes, in Hong Kong on September 11," the Hong Kong edition of the China Daily said on its Facebook page alongside a picture of the attacks on the twin towers in New York.

"The 9/11 terror plot also encourages indiscriminate attacks on non-native speakers of Cantonese and starting mountain fires."

The post said "leaked information was part of the strategy being schemed by radical protesters in their online chat rooms".

The former British colony of Hong Kong returned to China in 1997 under a "one country, two systems" formula that guarantees freedoms not enjoyed on the mainland, including an independent legal system, triggering the anger over the extradition bill.

Hong Kong leader Carrie Lam has said she will withdraw the bill but many Hong Kong residents fear Beijing is steadily eroding the autonomy of the Asian financial hub.

China denies meddling and has accused the United States, Britain and others of fomenting the unrest.

"We don't even need to do a fact check to know that this is fake news," said one protester, Michael, 24, referring to the China Daily post.

"The state media doesn't care about its credibility. Whenever something they claimed to have heard on WhatsApp or friends' friends, they will spread it right away."

The protesters called off action on Wednesday.

"In solidarity against terrorism, all forms of protest in Hong Kong will be suspended on Sept. 11, apart from potential singing and chanting," they said in a statement.

The China Daily report was worrying, said another protester, Karen, 23.

"When they try to frame the whole protest with those words, it alarms me," she said. "They are predicting rather than reporting. I think people calling it off today is a nice move."

Lam said in a speech on Wednesday that Hong Kong was grappling with significant challenges, from the trade dispute between China and the United States to the recent unrest.

"My fervent hope is that we can bridge our divide by upholding the one country, two systems principle, and the Basic Law, and through the concerted efforts of the government and the people of Hong Kong," she told business leaders.

The Basic Law is Hong Kong's mini-constitution.

The protests spread to the sports field on Tuesday, as many football fans defied Chinese law to boo the national anthem ahead of a soccer World Cup qualifier against Iran.

Several peaceful protests are planned for the next few days, combining with celebrations marking the Mid-Autumn Festival.