Colombo: Sri Lanka's new president-elect Maithripala Sirisena was a low-profile minister until he emerged as the best hope for a fractured opposition to topple South Asia's longest-serving leader, Mahinda Rajapakse. It is a stunning turnaround for the mild-mannered 63-year-old, who became an unlikely rallying point for disaffected Sri Lankans when he walked out of Rajapakse's government a day after sharing dinner with the strongman president.

His pledge to root out official corruption and restore the independence of the judiciary tapped into a simmering resentment over what many saw as Rajapakse's increasingly authoritarian rule. "From tomorrow, we will usher in a new political culture," he said as he cast his vote on Thursday. "There will be peace and rule of law under my presidency."

Another key pledge was to "end the Rajapakse family rule", after the president packed the corridors of power with his relative. Like Rajapakse, the former health minister is a Buddhist from the majority Sinhalese community, but many voters appear to have chosen him because of who isn't, not because of who he is. He secured the support of many of Sri Lanka's minority groups, including the Tamils, because he offered a credible alternative to the long-time leader.

Former colleague Austin Fernando described Sirisena as a "mild-mannered, soft-spoken politician. "He is unabrasive. A likeable chap who can easily command respect," said Fernando, a retired senior civil servant. The decision to turn against the a powerful president demonstrated a steeliness that belies that mild reputation.

Former army chief Sarath Fonseka, who mounted a failed bid to challenge Rajapakse in 2010, was jailed for over two and a half years on controversial charges and through an even more contentious legal process. "I know what happened to General Fonseka can happen to me too," Sirisena said at the start of the campaign.

Dressed in the white sarong and tunic favoured by Sri Lankan politicians, Sirisena appeals to a rural electorate while his main backer, the centre-right United National Party (UNP), is more popular in urban areas. He has pledged to reform the presidency within 100 days, abolishing many of its executive powers and returning the country to a Westminster-style parliamentary democracy where the police, the judiciary, and the civil service are independent institutions.

The son of a World War II veteran, Sirisena entered parliament in 1989 after settling in the eastern district of Polonnaruwa. He was a soft target for the Tamil Tiger rebels during the height of fighting and says the separatists may have tried to assassinate him on at least five occasions. He was jailed for nearly two years after being arrested on suspicion of leading a revolt against the government in 1971 when he was just 20.

After his defection from the government, Rajapakse kicked Sirisena out of the Sri Lanka Freedom Party although he has said he is still a member. His vision for the country ties in closely with the free-market, investor-friendly policies of the opposition UNP which provided him with the political base to challenge Rajapakse.