Most of Hong Kong's seven million people will not get to vote for their city's leader but the contest could stir debate on democratic reforms and pave the way for future contests.
Although pro-democracy groups and Hong Kong citizens have been clamouring for universal suffrage, China's parliament decided against that in April 2004.
"We started off thinking that we could not possibly get the entry ticket, now we got already 111 nominations," said Leong.
Incumbent Tsang is expected to win by a 
landslide in the March 25 ballot [GALLO/GETTY]
"This is a historic moment for Hong Kong." No candidate from Hong Kong's pan-democratic camp had managed previously to obtain the minimum 100 nominations required to get on the ballot sheet.
Leong's nomination on a pro-democracy ticket takes Hong Kong politics into uncharted waters.
Beijing is more accustomed to seeing its candidates sail into office unopposed.
Leong described the last three months of campaigning as a "miraculous" democratic journey and said he hoped his candidacy would help Hong Kong realise direct elections by 2012.
Tung Chee-hwa, Hong Kong's former chief executive, won unchallenged in 2002, as did Tsang, who stepped in after Tung resigned abruptly in 2005, citing health problems.
Tsang was formally elected in June 2005. Leong, who won a directly elected seat in the city's legislature in 2004, is a founding member of the Civic party, a rising force in Hong Kong party politics.
Timothy Wong, a political analyst at Hong Kong's Chinese University, said the contest provided Leong a "good opportunity" to accumulate more experience.
"It will strengthen his party, and promote the party's image," he said.