Mahinda Rajapaksa staged a dramatic political comeback on Friday, becoming Sri Lanka's new Prime Minister after President Maithripala Sirisena sacked premier Ranil Wickremesinghe.

Visuals of the former president Rajapaksa taking oath as the premier were released to media and was shown on TV channels. Rajapaksa, 72, tweeted a photo of him and Sirisena after the swearing in ceremony. "Former #SriLanka President Mahinda Rajapaksa sworn in as new Prime Minister," he said in the tweet.

The sudden development came after Sirisena's broader political front United People's Freedom Alliance (UPFA) announced that it has decided to quit the current unity government with prime minister Wickremesinghe's United National Party (UNP).

Mahinda Amaraweera, agriculture minister and the general secretary of the UPFA, told reporters that the UPFA decision has been conveyed to Parliament.

The development triggered an angry reaction from Wickremesinghe, who asserted that the swearing in of Rajapaksa was "illegal and unconstitutional".