During the vote, supporters clapped while electors from the democratic camp chanted slogans for universal suffrage in 2012, when the next chief executive election is scheduled.
Several dozen people staged a protest outside the convention centre where the electors were voting, waving banners and shouting, held back by a line of police.
"I think we have achieved something"
To get into the voting hall, electors had to run a gauntlet of protesters shouting at them to cast a blank ballot.
'Power to the people'
Pro-democracy legislator Leung "Long hair" Kwok-hung said: "I demand the [Special Administrative Region] SAR government return the power to the people".
Tsang's challenger Leong had no chance but he said his appearance on the ballot and televised policy debates with Tsang - both firsts for Hong Kong - had changed the political landscape permanently.
Leong is the first ever opposition candidate on a pro-democracy ticket to qualify for a leadership election since the handover in 1997.
"I think we have achieved something," he said on the way into the voting venue.
Leong's candidacy put issues such as universal suffrage, the worsening environment and education spending into the spotlight in a way an uncontested election could not have.
Leong tried to elicit a commitment from Tsang to have the next chief executive picked directly by the people in 2012, but the incumbent said he would have a plan for universal suffrage by then.
'One country, two systems'
Since Hong Kong returned to Chinese rule in 1997, the territory has been ruled under the principle of "one country, two systems" which gives it a significant degree of autonomy from the mainland.
But regular displays of political muscle-flexing by Beijing have worried democrats who fear that promises of greater democracy in the future are little more than empty words.
In 2004, for example, the Chinese government ruled that Hong Kong would have to obtain approval from Beijing to make changes to its electoral laws, including moves towards the direct election of its leader.
Martin Lee, a leading Hong Kong democrat, has described the 2004 ruling as Hong Kong's "darkest hour".
In pre-election briefings, Tsang said he favours an open democratic system for Hong Kong, and says he will deliver a full road map for democracy if he is picked.
|Alan Leong says he is looking to change |
Hong Kong's election culture [AFP]
"We all want to achieve universal suffrage as soon as we possibly can," he said.
But he said Hong Kong should be allowed to decide on its own what is the best system for the territory.
"I am not doing it because someone badgers me to do it from elsewhere," he said.
Under article 45 of Hong Kong's post-handover constitution, known as the Basic Law, the selection of the chief executive by universal suffrage should be the "ultimate aim".
But the document lays down no timescale as to when that should happen.
In the decade since the handover, Hong Kong has proved wrong the doubters who said the transition to Chinese rule would spell the end of its wealth and power.
The territory's booming economy has bounced back from recession and the impact of the Sars virus, backed up by a respected rule of law and one of Asia's most corruption-free administrations.
At the same time, Hong Kong has seen growing calls for reform and greater democracy.
|Incumbent Donald Tsang has promised to |
produce a road map to democracy [AFP]
For pro-democracy campaigners the watershed moment came on July 1, 2003, when half a million people marched through the city in protest against the then chief executive, Tung Chee-hwa, and proposed anti-subversion laws.
The laws were eventually scrapped and Tung resigned his post in March 2005.
Now, for the first time, an opposition candidate has gained the required backing of at least 100 members of the electoral college to get his name on the ballot.
In the run up to Sunday's vote, Tsang and Leong were both out on the stumps, laying out their policies for Hong Kong's future and even engaging in two televised debates.
That has given the election of Hong Kong's chief executive - normally little more than a bureaucratic formality – at least some of the flavour of an actual political campaign.