Sri Lanka's divisive Rajapaksa clan consolidated their grip on power on Thursday as the newly elected President swore in his brother as prime minister amid concerns they could roll back vital economic reforms.

Gotabaya and Mahinda Rajapaksa are credited with annihilating ethnic Tamil separatists to end Sri Lanka's civil war a decade ago when Mahinda was president and Gotabaya effectively ran the security forces.

Mahinda, the elder and more charismatic of the brothers, was unable to run for president, having already served the maximum of two terms between 2005 and 2015 - clearing the way for his sibling.

Thursday's nationally televised brief ceremony came after Ranil Wickremesinghe stepped down in the wake of Saturday's election.

Gotabaya, 70, was elected president on Saturday thanks to his popularity among the Sinhalese-Buddhist majority, thrashing his main moderate rival despite support from minority Tamils and Muslims.

Rajapaksa campaigned with promises to make the island of 21.6 million people safe in the wake of suicide bombings by homegrown Islamist extremists in April that killed 269 people.

The International Monetary Fund (IMF) said earlier this month that Sri Lanka's economy was slowly recovering from the impact of the Easter Sunday suicide bombings that crippled the booming tourism sector.