China's ambassador to London has accused foreign countries including the United States and Britain of interfering in Chinese internal affairs through their reactions to the violent clashes taking place in Hong Kong.

The Asian financial hub, which was handed over to China by former colonial ruler Britain in 1997 but enjoys a degree of autonomy under the "one country, two systems" formula, has been plunged into chaos for almost six months.

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In a dramatic escalation, Hong Kong police are laying siege to a university in Hong Kong, firing rubber bullets and tear gas to push back anti-government protesters armed with petrol bombs and other weapons to stop them from fleeing.

In London, ambassador Liu Xiaoming called a news conference at the Chinese embassy on Monday to comment on events in Hong Kong and criticise Western governments and media for their responses to the crisis.

"Some Western countries have publicly supported extreme violent offenders," he said.

"The US House of Representatives adopted the so-called Hong Kong Human Rights and Democracy Act to blatantly interfere in Hong Kong affairs, which are China's internal affairs.

"The British government and the foreign affairs committee of the House of Commons published China-related reports making irresponsible remarks on Hong Kong."

China has told the US and Britain to stop meddling in Hong Kong and accused them of taking sides. Credit: AP

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Liu also said that by criticising violent actions by the authorities as well as by the protesters, Britain was in effect taking sides.

"I think when the British government criticises Hong Kong police, criticises the Hong Kong government in handling the situation, they are interfering into China's internal affairs," he said.

"They look like they are balanced but as a matter of fact they are taking sides. That is our position."

The ambassador also attacked Western media, saying that reporting on Hong Kong was misleading and did not give enough prominence to violence perpetrated by the protesters.

He also dismissed Western media reports on the separate issue of what UN experts and activists condemn as repression in China's western Xinjiang region as "pure fabrication".

Australia has also expressed concern about the escalating violence in Hong Kong and the issue of the Uighurs.

As the ambassador's news conference was unfolding, the British Foreign Office issued the latest in a series of statements about Hong Kong.

"The UK is seriously concerned by the escalation in violence from both the protesters and the authorities around Hong Kong university campuses," a Foreign Office spokesman said.

"It is vital that those who are injured are able to receive appropriate medical treatment, and that safe passage is made available for all those who wish to leave the area. We need to see an end to the violence, and for all sides to engage in meaningful political dialogue ahead of the District Council elections on Sunday."

Also during the news conference at the embassy, a spokesman for Prime Minister Boris Johnson called on all sides to show restraint.

The European Commission on Monday also called on law enforcement authorities to keep their action "strictly proportionate".