Mr Sirisena, a longtime political insider – and an ally of former president Mahinda Rajapaksa until just a few weeks ago – won the election by capitalising on Mr Rajapaksa’s unpopularity among the island nation’s ethnic and religious minorities, as well as grumbling among the Sinhalese majority about his family’s growing power.
“With this victory we will implement the 100-day programme in our election manifesto,” Mr Sirisena told jubilant crowds in Colombo after his swearing-in.
Mr Sirisena had promised to change Sri Lanka’s constitution to drastically reduce the power of the president and return the country to a parliamentary system with a prime minister as its leader.
Mr Sirisena also promised that he would not run again for president.
He thanked Mr Rajapaksa for conceding defeat but called for future campaigns to be “much more mature”, and blasted the state media for its coverage.
“Even though they carried out character assassination and vilified me, I can say I had the maturity to bear it all as a result of my long political experience,” he said.
He took the oath of office with senior supreme court justice Kanagasabapathy Sripavan, bypassing the country’s chief justice, who was installed by Mr Rajapaksa in a widely criticised move to expand his authority even more.