Thousands of protestors chanting slogans and blaring vuvuzelas faced off Wednesday as Hong Kong legislators debated a controversial plan to enact limited political reforms in the Chinese territory.

Police surrounded the legislature with barricades and scores of officers kept pro-democracy and pro-Beijing activists apart as lawmakers inside thrashed out the plan, which would give voters a bigger say in Hong Kong's running.

After weeks of political manoeuvring, the reform plan is likely to be adopted by the legislature although critics complain it does not go far enough and are calling for full democracy for the city of seven million people.

"Officials say the vote will mark a historic moment for Hong Kong. But how come Hong Kong people are not involved in this historic moment? How come they are not consulted?" pro-democracy lawmaker Lee Cheuk-yan said.

The vote was adjourned to Thursday after the debate dragged on late into Wednesday evening.

Lawmakers in support of the plan were mobbed by pro-democracy campaigners as they left the Legislative Council building escorted by police officers after the meeting.

"Shameful! Shameful!" the protesters shouted, pointing their fingers and throwing waste paper at the legislators.

Hong Kong, which was handed over by Britain in 1997, has a legal and administrative system independent from mainland China but its constitutional development falls under communist Beijing's control.

While the financial and trading hub in southern China has some direct democracy, power ultimately rests in the hands of a Beijing-backed chief executive and business elites.

The reform plan has split the opposition camp. Moderates from the Democratic Party, whose support looks set to ensure the plan's passage, were condemned by harder-line lawmakers who want a firm timetable for universal suffrage.

"There was an under-the-table deal between the Democratic Party and Beijing. The party has betrayed Hong Kong people," said Wong Yuk-man, leader of the League of Social Democrats.

The Democratic Party, the city's largest and oldest pro-democracy group, dismissed the talk of betrayal and argued the latest plan was a step forward.

Emily Lau, a member of the party, said: "If people feel that I have departed from my original stance of fighting for universal suffrage in 2012, I will accept the criticisms.

"But I urge you to focus on our long-term objective — we will continue our fight for universal suffrage."

The plan has also threatened to divide the Democratic Party, with veteran Andrew Cheng announcing that he would quit the group as he intended to vote against the plan.

In a U-turn on Monday, the government said all 10 new seats that it proposes to add to Hong Kong's Legislative Council would be directly elected, a concession that won over the Democratic Party.

Only half the current 60-seat legislature is directly elected, with the rest picked by "functional constituencies" based on professions and mainly comprising pro-Beijing elites.

Previously, the government had proposed to split its planned 10 new seats between directly elected and functional constituencies.

The city's leader is meanwhile chosen by an election committee made up of 800 members handpicked by Beijing. The government is proposing to increase the number of committee members to 1,200.

Police prepared for the possibility of clashes between the more radical pro-democracy and pro-Beijing factions gathered outside in heavy rain.

"We hope there will be no need to use force, but when necessary, we will deploy sufficient manpower and consider what strategy to adopt to maintain order," a police spokeswoman told AFP.

Protestors demanding one-person-one-vote held banners saying "Reject the devil's proposal", and made a deafening noise on the vuvuzela horns made famous by crowds at the football World Cup in South Africa.

Beijing has said that, at the earliest, universal suffrage can be introduced for the election of Hong Kong's chief executive in 2017 and for the legislature in 2020.

That would augment freewheeling Hong Kong's status as a constitutional curiosity within China, which as a whole is controlled with an iron fist by the communist party.

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