Former Sri Lankan President Mahinda Rajapaksa has been sworn in as the new prime minister of the country on Friday, after President Maithripala Sirisena in a surprise move sacked Ranil Wickremesinghe that now threatens political turmoil in the island nation.

In a video, it is evident that the former President has been appointed as the Prime Minister of the Democratic Socialist Republic of Sri Lanka and was sworn in before President Maithripala Sirisena at the Presidential Secretariat on Friday, surrounded by MPs from the Joint Opposition and Ministers and MPs of the Sri Lanka Freedom Party (SLFP).

The unexpected announcement came just minutes after the United People's Freedom Alliance (UPFA) decided to withdraw from the unity government.

Sri Lanka's president issued a formal notice on Saturday for Ranil Wickremesinghe to step down as prime minister a day after he appointed Mahinda Rajapaksa as the PM.

However, Wickremesinghe continued to occupy Temple Trees, the official residence of the prime minister, and insisted in a letter to Sirisena that he was still in office.

He has said that he can be removed only by parliament where his party enjoys a majority, and also vowed to seek legal action against what he condemned as an unconstitutional move against him.

"I am addressing you as the prime minister of Sri Lanka," Wickremesinghe told reporters on Saturday.