Hong Kong lawmakers rejected a Beijing-backed political reform package on Thursday as pro-democracy legislators united to vote down a contentious electoral roadmap that has sparked mass protests.

Most pro-government lawmakers staged a walkout as the bill headed for defeat -- a move slammed as "farcical" by the opposition and which analysts said could be interpreted as a slap in the face for the current administration.

Just eight lawmakers cast their vote in support of the package, while 28 lawmakers voted against it, including all 27 pan-democrats.

The bill needed the support of two-thirds of the city's 70-strong legislature in order to pass.  

Although the Hong Kong government's plan would have given all residents the right to vote for the chief executive for the first time in 2017, it adhered to a Beijing ruling that candidates must be vetted by a loyalist committee.

The proposal was derided as "fake democracy" by opposition lawmakers and campaigners.

The pro-government camp was left in disarray after the walkout.

"We have no idea what happened with the rest of the group suddenly deciding to leave the chamber," said James Tien of the pro-government Liberal Party, whose members stayed to vote for the bill.

The lawmakers left after their request for a 15-minute suspension was rejected by the speaker.

Pro-government legislator Jeffrey Lam said they had requested the delay so that another lawmaker, who had been ill and was running late, could join the vote.

Civic Party leader Alan Leong said that it would "go down in history" that only eight people supported the bill.  

"The message that we are sending to the central people's government and the Hong Kong government is that Hong Kong people do not want to take on this fake democratic package," said Leong.

"We do not want our votes to be used to legitimise an anointment of a chief executive that belongs and is accountable to only vested interests."

Around 100 pro-democracy supporters outside the legislature cheered, punched the air and hugged each other as the result was broadcast live on a big screen.

"I'm excited!" said Ken Tsang, 30, who works in the IT industry.

"An undemocratic proposal should not be passed. It was a proposal that would only benefit those in power."

Hundreds of pro-government supporters, who had been gathered throughout the morning, shouted through loudspeakers to "vote out" the democrats.

"I'm very disappointed that I cannot have my vote (for leader)," said Leung Ting-to, 63, who was standing with the pro-Beijing camp.