More cunning than the 'old fox': Sri Lanka's acting president
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He is only head of state in an acting capacity after Gotabaya Rajapaksa resigned in disgrace after fleeing to Singapore, but the position is one Wickremesinghe has sought for decades.
A few families have long dominated politics in the Indian Ocean island nation, and Wickremesinghe is the nephew of one its longest-serving leaders, Junius Jayewardene, who was in power for 12 years until stepping down in 1989.
Dubbed the "old fox", Jayewardene was renowned for his cunning, but his nephew is regarded as an even shrewder navigator of the country's internecine power networks.
It was Jayewardene who brought him into politics by making him a deputy foreign affairs minister in 1977. Commentators joked the initials of their United National Party (UNP) actually stood for Uncle and Nephew.
Family members say that Jayewardene, who died in 1996, had wanted to ensure that Wickremesinghe becomes president "even for one day".
Now he will hold the position for at least six days, with parliament due to elect Rajapaksa's long-term successor on Wednesday -- although Friday's swearing-in means Wickremesinghe maintains his record of never having fulfilled a full term as prime minister.
He ran for the presidency twice -- in 1999 and 2005 -- losing both elections, and the UNP was annihilated in a parliamentary election in 2020, leaving Wickremesinghe as its only MP.
But his political manoeuvring enabled him to outfox opponents and secure his sixth appointment to the premiership earlier this year after Rajapaksa's brother Mahinda resigned.
Wickremesinghe is married to Maithree, an English lecturer. They have no children and have bequeathed their assets to his old school and their universities.
But their impressive library of more than 2,500 books -- which he called his "biggest treasure" -- was among the losses when their house was torched last week by demonstrators who also drove Rajapaksa from his official residence.
Born into a wealthy as well as politically connected family rooted in publishing and plantations, Wickremesinghe started work as a rookie reporter at one of the family newspapers.
But he turned to a legal career after the family firm was nationalised in 1973 by Sirima Bandaranaike, the world's first woman prime minister.
"If Lake House had not been taken over, I would have become a journalist. So actually, Mrs Bandaranaike sent me to politics," Wickremesinghe once told AFP.
His first appointment as prime minister was as a result of the May 1993 assassination of president Ranasinghe Premadasa by a suicide bomber.
The then premier Dingiri Banda Wijetunga was elevated to the presidency, and picked Wickremesinghe -- then Industry, Science and Technology minister -- to replace him.
A similar attack arguably denied him the presidency six years later: his main election rival Chandrika Kumaratunga was wounded by a suicide bomber just three days before the polls.
She brought the nation to tears in a television appearance with a patch over the right eye she had lost and received a significant sympathy vote, with Wickremesinghe losing an election many thought he would win.
Now the political wheel may turn once more: the demonstrators who ousted Rajapaksa are also demanding Wickremesinghe's departure, and Premadasa's son Sajith is one of the leading contenders to be elected president next week.
Wickremesinghe long had a "Mr Clean" image, but it was muddied during his last-but-one prime ministerial term in 2015-19 when his administration was rocked by an insider trading scam involving central bank bonds.
His schoolmate and choice as central bank chief was a key accused, raising allegations of cronyism.
Wickremesinghe was also accused of protecting members of the Rajapaksa clan who have been accused of graft, kickbacks, siphoning off public finances and even murder.
He takes charge of a bankrupt nation that has defaulted on its $51-billion foreign debt and without money to import essential goods.
His status as a pro-West, free-market reformist could smooth bailout negotiations with the International Monetary Fund and foreign creditors, but he has already warned there will be no quick fix to the nation's unprecedented economic woes.
"The worst is yet to come. We have very high inflation now and hyperinflation is on its way," Wickremesinghe told parliament last week. "We are bankrupt."