Britain will not ignore the treatment of protesters in Hong Kong, UK Foreign Minister Dominic Raab has said, after violence again erupted between police and pro-democracy protesters.

Key points: Mr Raab made the comments at the Conservative Party's annual conference

Mr Raab made the comments at the Conservative Party's annual conference Sunday saw some of the most violent clashes since the protests began

Sunday saw some of the most violent clashes since the protests began Hong Kong singer and activist Denise Ho was attacked by a masked man in Taiwan

Mr Raab made the comments on Sunday as violence erupted in Hong Kong for the second consecutive day, throwing the city's business and shopping belt into chaos and sparking fears of more ugly scenes leading up to China's National Day this week.

"We won't look the other way when the people of Hong Kong are beaten indiscriminately on commuter trains for exercising the right to peaceful protest," Mr Raab told the Conservative Party's annual conference.

Police in riot gear beat protesters

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 21 seconds 21 s Dominic Raab said protesters in Hong Kong were being "beaten indiscriminately".

Hundreds of thousands of people have taken to the streets of Chinese-ruled Hong Kong since mid-June in protests denouncing what they perceive as creeping interference by Beijing, a charge China denies.

Sunday saw some of the most violent clashes since the protests began, with police in riot gear seen beating and detaining protesters near Hong Kong's government complex.

Police also fired blue liquid — used to identify protesters — from a water cannon truck and multiple volleys of tear gas, after demonstrators hurled Molotov cocktails at officers and targeted the government office building.

"We know that in the face of the world's largest totalitarian regime, to quote Captain America, 'Whatever it takes", Justin Leung, a 21-year-old protester, said.

"The consensus right now is that everyone's methods are valid and we all do our part."

Protesters are planning to march again on Tuesday despite a police ban, raising fears of more violent confrontations that could embarrass Chinese President Xi Jinping as his ruling Communist Party marks 70 years since taking power.

Protesters threw petrol bombs during clashes with riot police. ( AP: Vincent Yu )

Posters are calling for October 1 to be marked as "a day of grief".

Hong Kong's Government has already scaled down the city's National Day celebrations, cancelling an annual fireworks display and moving a reception indoors.

Hong Kong activist attacked in Taiwan

In Taiwan, Hong Kong singer and activist Denise Ho was attacked a masked man who threw red paint at her during a rally that attracted more than 100,000 people.

Ms Ho, who earlier this month urged members of the US Congress to pass legislation to combat human rights abuses in Hong Kong, was talking to reporters when the man ran up to her and poured red paint over her head.

Hong Kong activist Denise Ho spoke out about human rights in Hong Kong in the US earlier this month. ( AP: Pablo Martinez Monsivais )

Two Taiwanese men were arrested immediately after the attack, the island's Crime Investigation Bureau said, adding that the pair were linked to an organised crime group that supports closer ties between self-ruled Taiwan and China.

"A lot of social activists in Hong Kong are actually subjected to situations like this every day. I think this is very obviously a sort of suppression and intimidation," Ms Ho said.

"I think that Hong Kong people will not back down or be scared by situations like this," she said, calling the incident "an attack on free speech".

Taiwan President Tsai Ing-wen said Ms Ho's attackers "will be harshly punished" according to the law.

"Please take our will and determination to protect Taiwan's democracy seriously," Ms Tsai said in a post on Facebook.

"This is the land of freedom and civilization, not the territory of rampant totalitarianism. Don't try to challenge Taiwan's democracy and rule of law."

People in Taiwan have been closely watching Hong Kong and many have become increasingly wary of Beijing's "reunification" agenda for Taiwan.

China considers democratic Taiwan a breakaway province to be taken under its control, by force if necessary, and has suggested an arrangement similar to Hong Kong's "one country, two systems" formula guaranteeing certain freedoms.

Reuters/AP