The UK has warned China there will be "serious consequences" if the two states' agreement on Hong Kong is not honoured.
Britain has warned China there would be serious consequences if the Sino-British declaration on Hong Kong was not honoured, saying Britain stood behind the people in the former British colony.
China has condemned violent demonstrations in Hong Kong as an "undisguised challenge" to the "one country, two systems" formula under which the city is ruled, which allows freedoms not enjoyed in mainland China.
"The UK signed an internationally binding legal agreement in 1984 that enshrines the 'one country, two systems rule', enshrines the basic freedoms of the people of Hong Kong and we stand four square behind that agreement, four square behind the people of Hong Kong," foreign minister Jeremy Hunt said.
"There will be serious consequences if that internationally binding legal agreement were not to be honoured."
Protesters, angry about proposed legislation allowing extraditions to mainland China, smashed their way into Hong Kong's legislature on Monday, with police using tear gas to disperse them.
Hunt urged the Hong Kong authorities that any repression would exacerbate people's concerns.
"We in the UK condemn violence on all sides, and many people who strongly support the pro-democracy demonstrators in Hong Kong will have been deeply dismayed by the scenes they saw on TV last night," Hunt said.
"But, we urge the authorities not to use what happened as a pretext for repression, but rather to understand the root causes of what happened which is a deep-seated concern by people in Hong Kong that their basic freedoms are under attack."