Hong Kong's pro-democracy camp has secured about a quarter of the seats in the Election Committee that will pick the city's next leader.

In the Sunday election, the pan- democrats won 325 seats - their biggest number ever - in the 1,194-member panel, which will pick the city's next Chief Executive on March 26 next year. The pan-democrats are the main opposition to the pro-establishment bloc deemed to be supportive of China.

A record 107,000 voters, or 46 per cent of 230,000 eligible voters, turned up to vote to fill 733 seats in the committee. The remaining 461 seats were returned uncontested or held by, for instance, lawmakers.

The turnout was nearly 20 percentage points higher than that for the previous committee election in 2011, when the overall turnout rate was 27.6 per cent. Analysts said this reflected professionals' discontent with the city's government.

Political analyst Willy Lam believed Hong Kong Chief Executive Leung Chun Ying's announcement last Friday that he would not seek re-election spurred more pro-democracy supporters to vote.

"Pan-democrats want to prove to Beijing that they have the ability and they are a force to be reckoned with," said Dr Lam, who noted that the ballots were previously dominated by pro-Beijing voters.

The Election Committee is made up of four major sectors - economic, professional, social and political - each having some 300 seats. These are subdivided into 38 sub-sectors.

The pan-democrats, who won 205 Election Committee seats in 2011, had targeted at least 300 seats in their bid to stop Mr Leung from winning a second term. At least 601 votes are needed for a contender to become the next Chief Executive.

Occupy Central co-founders, who had failed in their push for a "one man, one vote" system in choosing the Chief Executive four years ago, managed a clean sweep of 30 seats in the higher education sub-sector.

The pro-democracy camp saw landslide victories in five other professional sub-sectors - legal, education, social welfare, IT and health services. It also gained almost all seats in the accountancy and the architectural sub-sectors.

Veteran politician Alan Leong of the Civic Party, who was elected under the legal sub-sector, said that by acting together, pan-democrats could prevent another candidate who showed similar traits and ideologies as Mr Leung from being elected as the next Chief Executive.

"Our unity means we are not a negligible force. But exactly how important we shall be depends on the final mix of candidates."

So far, only retired judge Woo Kwok Hing has announced he is running for the top post. A pro-establishment figure, lawmaker and former security secretary Regina Ip, is expected to announce her candidacy on Thursday. Two other potential pro-establishment candidates are Chief Secretary Carrie Lam and Financial Secretary John Tsang, who has quit his post and is expected to declare his candidacy.

If all three potential pro-establishment contenders enter the race, it could lead to votes being split, giving the pan-democrats a bigger say in the outcome, said analysts.

"We could see (Mr) Leung's supporters turning to Carrie Lam and there is a possibility that John Tsang might win some votes from the pan-democrats," said Dr Lam.

He added there might even be two rounds of election if no one secures more than 600 votes.