China issued a formal complaint to Britain on Wednesday after Deputy Prime Minister Nick Clegg met pro-democracy activists in Hong Kong campaigning against Beijing's tightening control of the former British territory.
Hong Kong, reverted to Chinese rule in 1997 under a "one-country, two-systems" deal ensuring it considerable autonomy as a capitalist hub.
But pro-democracy campaigners have warned of the erosion of the city's freedoms and are pushing for the direct election of its leader by universal suffrage in 2017.
British media reported that veteran activists Anson Chan and Martin Lee met Clegg on Tuesday. Clegg has criticised Prime Minister David Cameron for prioritising trade with the world's second largest economy over democracy in Hong Kong, media said.
Asked about the meeting, Chinese Foreign Ministry spokesman Hong Lei said China was firmly opposed to any foreign interference in its internal affairs on any pretext.
"The UK's actions are an interference in China's internal affairs. China expresses strong dissatisfaction," Hong said in a statement posted on the ministry website.
"China urges the UK-side to earnestly respect China's stance and concerns, abide by its promises... and take practical efforts to safeguard China-UK relations and bilateral cooperation," Hong said.
Chan, a former senior Hong Kong official, and Lee, one of the founders of the Hong Kong opposition Democratic Party, met US Vice President Joe Biden at the White House in April, in one of the most high-profile attempts to flag Hong Kong's democracy concerns internationally.
While Beijing says Hong Kong can go ahead with a vote in 2017 for the city's leader, Hong Kong's mini-constitution, the Basic Law, specifies that only a nominating committee can pick leadership candidates.
Democracy activists fear that the committee will essentially keep opposition candidates out of the running by stacking the poll with Beijing loyalists.
The extent of democratic reform in Hong Kong is increasingly becoming a flashpoint. The standoff intensified last month when Beijing published an unprecedented cabinet-level white paper bluntly reminding Hong Kong that China remained in control.
Hundreds of thousands of people marched through Hong Kong on July 1 in support of full democracy, demonstrations that followed an unofficial referendum on democracy in which nearly 800,000 voted.