A by-election on Sunday to fill one seat in Hong Kong's 60-member legislature pits two celebrity politicians against each other, energising debate on democratic reform in a city that returned to Chinese rule 10 years ago.

Hong Kong's fractious pro-democracy camp is looking for a symbolic boost from former civil servant Anson Chan after a demoralising drubbing in this month's district council elections at the hands of the city's biggest pro-Beijing party.

Chan, 67, made a name for herself as the first Chinese, first female head of the Hong Kong civil service under British rule. She is a popular public figure whom backers have dubbed "Hong Kong's conscience" for her strong support for civil liberties.

Former Hong Kong Chief Secretary Anson Chan attends an election campaign at Aberdeen in Hong Kong November 27, 2007, ahead of Sunday's pool for the Legislative Council. REUTERS/Bobby Yip

A win by Chan, who led in recent polls, could give the democratic camp some momentum ahead of a five-yearly legislative election in 2008, analysts say. The democrats are already expecting possible losses.

"At the moment the political climate is not very favourable to the pro-democracy camp," said Cheung Chor-yung, an expert at City University of Hong Kong.

In the other corner is Regina Ip, 57, another former civil servant who enjoys the support of the pro-establishment constituency but is perhaps best known for trying to ram an unpopular anti-subversion law through the legislature in 2003 when she was security chief.

Ip's zealous bid to pass that law is widely blamed for sparking a protest that drew half a million people into the streets, shocking leaders in Beijing into reassessing their approach to the territory.

A pivotal campaign issue has been universal suffrage.

Britain handed back Hong Kong to Communist-ruled China in 1997 with the guarantee of a high degree of autonomy. The city has maintained its free-wheeling ways of life without universal suffrage, an idea the British promoted only in the dying days of colonial rule.

Hong Kong's leader is picked by an 800-seat committee under the influence of Beijing, and half the legislature is popularly elected. The city's constitution says universal suffrage is the goal, but is vague on the timing.

Chan wants full democracy by the next election for chief executive, in 2012, and says delay would be unacceptable. Ip is happy to leave the decision on timing to Beijing, which few expect to give the green light to universal suffrage before 2017.

Although she leads the polls, not everyone in the pro-democracy camp is at ease with Chan, who is seen as a relative newcomer to the movement having emerged from retirement a year ago to join calls for early universal suffrage.

Many took it as a bad sign in October when she left a democracy rally early to visit her hairdresser, making headlines.

"Far, far more than I expected have reservations about Anson as a candidate, and that certainly may affect her performance," said Cheung. Ip's pro-establishment camp, on the other hand, has "better solidarity, clearer objectives".

"This is going to be close," said analyst Ma Ngok, of the Chinese University of Hong Kong.