Sirisena had called for January polls after it was clear Rajapaksa wouldn't command majority in legislature.
Colombo: Ousted Sri Lankan prime minister Ranil Wickremesinghe's United National Party will challenge in court what it termed as the illegal dissolution of Parliament by President Maithripala Sirisena a day after he announced a snap election in January.
"The party will be pursuing the intervention of the courts to ensure the rule of law and the Constitution is protected against autocratic moves," a UNP statement said.
Former finance minister and UNP leader Mangala Samaraweera said, "the emerging tyranny of Maithripala Sirisena will be fought in courts, in parliament and at the election".
Sirisena sacked Wickremesinghe on October 26 and on Friday he set the election for January 5.
Sri Lanka's political crisis deepened as Sirisena dissolved Parliament and announced snap polls after it became evident that he did not have enough support in the House for Prime Minister Mahinda Rajapaksa, who was appointed by him under controversial circumstances.
Dinesh Gunawardena, a government minister from Sirisena's party alleged that parliament was dissolved because of the conduct of Speaker Karu Jayasuriya.
"He gave illegal orders in defiance of the president," Gunawardena said referring to Jayasuriya's assertion that parliament must hold a floor test to prove the legality of the appointment of Rajapaksa as prime minister replacing Wickremesinghe.
Sirisena had suspended the House until November 16 which was later advanced by two days following international pressure. Western nations had urged Sirisena to uphold the law and abide by parliamentary procedure.
Former president Rajapaksa commenting on Sirisena's decision said, "a general election will truly establish the will of the people and make way for a stable country".
Marxist JVP's senior member Vijitha Herath said that all must unite to protect the basic law of the country.
"We appeal to everyone to get together to safeguard democratic rights and the basic law of the country," he said adding they would take part in a broader alliance to fight the "injustice" at the election.
Sirisena signed a gazette notification to dissolve Parliament with effect from Friday midnight, in another surprise move that comes after two weeks of political and Constitutional turmoil.
According to the gazette notice, nominations to contest the snap election would be taken between November 19 and 26. The election will be held January 5 and the new parliament would be convened on January 17.
Sri Lanka's Parliament stands dissolved some 21 months ahead of its schedule in August 2020, amid a worsening Constitutional crisis triggered by the surprise sacking of Wickremesinghe.
The dissolution came hours after a close aide of the president said that Sirisena had decided that there will be no snap election or a national referendum to end the political turmoil.