The sudden development came amid growing tensions between Sirisena and Wickremesinghe on several policy matters and the President has been critical of the Prime Minister and his policies, especially on economy and security.
The change in government was carried out "in line with the constitution" and he would not backtrack on it, "whatever the pressure comes", the president said.
President Sirisena said he was happy to be on the same poitical platform with Prime Minister Mahinda Rajapaksa after exactly four years.
"I wish to emphasize that I am compelled to accept the status that existed previously until such time that they (the pro-Wickremesinghe lawmakers) and the new political alliance prove their majority in Parliament", he said.
This follows a warning from the European Union that it could withdraw duty-free concessions for Sri Lankan exports if it didn't stick to commitments on national reconciliation.
Asked by The Sunday Morning if he had the full support of his own party (UNP), Wickremesinghe confirmed that he commanded its full backing.
Busloads of men and women from around the country descended on the capital as Rajapakse thanked President Maithriapala Sirisena for sacking Ranil Wickremesinghe and appointing him in his place.
However, Wickremesinghe said the Police had launched an investigation into the allegations and that it was being seriously looked into.
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In a dramatic turn of events last month, Rajapaksha became the new Prime Minister of the Indian Ocean island nation after Sirisena sacked premier Wickremesinghe who termed the move as "unconstitutional" and vowed to prove his majority in Parliament.
In a statement after he was sworn in, Rajapaksa said he wanted to end religious and ethnic divisions.
Foreign Minister Sarath Amunugama told a news conference Jayasuriya was not being impartial.
Also, President Sirisena's move to join Rajapaksa and inviting him back into government counters the promises he made including the investigation into the former government's actions during the civil war. "The public must protest the way the MPs are being brought for money", Abeywardena said. Sirisena has also accused Wickremesinghe's government of not taking seriously an alleged assassination plot against him.
Pradeep Kariyawasam, the head of Mr Rajapaksa's party in Colombo, said that although Mr Wickremesinghe continues to maintain he is the country's legitimate leader, "the people are with us".
The former strongman remains a popular if polarising figure in Sri Lanka politics.