China on Wednesday accused London of interfering in its internal affairs after British Deputy Prime Minister Nick Clegg met two leading Hong Kong pro-democracy activists urging greater freedoms from Beijing.

China "lodged solemn protests" with Britain over Tuesday's meetings with Martin Lee, founder of Hong Kong's opposition Democratic Party, and Anson Chan, the former number two in the city's government, the official Xinhua news agency reported.

"What Britain has done is interference in China's internal affairs. China strongly opposes it," said Beijing's foreign ministry spokesman Hong Lei.

"Hong Kong's affairs fall within China's internal affairs. China firmly opposes any interference in Hong Kong affairs by any country under whatever pretext," said Hong.

Discontent in the former British colony, which was handed back to China in 1997, is at its highest level in years, notably over Beijing's insistence that it vet candidates before the vote for the city's next leader in 2017.

Under the "one country, two systems" deal at the time of the handover, the semi-autonomous city has guaranteed liberties not seen on the Chinese mainland, including freedom of speech and the right to protest.

But concerns are growing that these freedoms are being eroded.

A string of attacks on media workers has raised fears for press freedom, while Beijing published a "white paper" last month on Hong Kong's future that was widely seen as a warning to the city not to overstep its bounds.

Chan and Lee appeared before a British parliamentary committee on Wednesday, urging London not to turn a blind eye to "attacks" on freedoms in its former colony.

The outspoken campaigners told the Foreign Affairs Committee that London had been failing in its responsibility towards democracy in Hong Kong.

"It is vital that the British government does not turn a blind eye to current developments in Hong Kong," Chan told the committee members.

The pair heavily criticised a British parliamentary report earlier this month in which then-British foreign secretary William Hague said the city's unique constitutional framework has worked well and that there was no "perfect model" for electoral reform.

Hong Kong's current leader Leung Chun-ying was chosen by a pro-Beijing committee.

An unofficial referendum on how the next leader should be chosen drew a massive turnout of almost 800,000 people last month, angering Beijing.

It was followed by a huge pro-democracy march on July 1 which organisers said was the biggest protest since the 1997 handover.