The Hong Kong government launched a legal bid to unseat four elected pro-democracy lawmakers Friday sparking accusations from the opposition camp that they are being subjected to a witch hunt.

It comes as concerns grow that Beijing is stepping up interference in the semi-autonomous city's politics after two lawmakers advocating a complete split from China were banned from taking up their seats in parliament for failing to take their oaths properly.

Baggio Leung and Yau Wai-ching deliberately misread their oaths of office, inserted expletives and draped themselves with "Hong Kong is not China" flags during a swearing-in ceremony in October.

They were banned from office by the city's High Court following a special "interpretation" of the city's constitution by Beijing that effectively prevented them from taking up their seats because of the way they took their oaths.

They lost an appeal against the court decision Wednesday.

The government is now challenging a group of more moderate pro-democracy legislators over their oath-taking, including the city's youngest ever legislator Nathan Law.

The four are not stridently pro-independence, but Law and teacher Lau Siu-lai have advocated self-determination for Hong Kong.

Law and Lau made their names as part of Hong Kong's mass pro-democracy "Umbrella Movement" rallies in 2014.

Both were part of a new wave of rebel lawmakers voted in to the legislature in September in citywide polls.

The government said in a statement late Friday that it had asked the court to declare the offices of the four lawmakers vacant.

"The government stressed that the commencement of the...proceedings was a purely legal and enforcement decision and did not include any political considerations," the statement said.

Law hit out at the government's move saying it was "a blow to all democratic forces".

"All democrats will stand firm together defending the dignity of all voters," he added.

Pro-democracy lawmakers protested outside government headquarters with a banner saying the city's pro-Beijing leader is "staging a coup and declaring a war on voters".

Hong Kong is semi-autonomous after being handed back to China by Britain in 1997 but there are fears that its liberties are disappearing.

Frustration among young campaigners over a lack of political reform has fuelled a new movement advocating independence and self-determination for the city.

The oath of office requires lawmakers to repeatedly describe Hong Kong as a "special administrative region of China".

Law quoted Gandhi before taking the oath, saying: "You will never imprison my mind", and used intonation to make his pledge sound like a question.

Of the other three named by the government, Lau read her pledge at a snail's pace.

Veteran anti-China lawmaker Leung Kwok-hung, known as "Long Hair" raised a yellow umbrella -- symbol of the democracy movement -- during his swearing in.

Lawmaker Edward Yiu added lines to his oath, saying he would "fight for general universal suffrage".

However all were officially sworn to the legislature, unlike Yau and Baggio who never successfully took their oaths.