Shortly before sacking the legislature, Sirisena also took over the police department by attaching it to his defence ministry.
The island nation was plunged into a political crisis after Sirisena sacked Prime Minister Ranil Wickremesinghe and replaced him with his former rival Mahinda Rajapaksa. The election will be held January 5 and the new parliament would be convened on January 17.
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"We fear that recent actions, if not corrected, will threaten your country's democratic development and derail the progress made in recent years", the three lawmakers said in a letter, a copy of which was obtained by the Associated Press. We need a free and fair election.
When Wickremesinghe refused to leave the post and demanded a vote in parliament to prove that he has a majority support in the House, the President prorogued the legislature until November 16 to allow the new Prime Minister to lure the MPs to his side with cabinet posts.
Rajapakse and Wickremesinghe have been battling for the prime minister's post for two weeks as worldwide concern grows over the mounting turmoil.
The United National Party (UNP) of the deposed Prime Minister, Ranil Wickremesinghe, and its coalition partners will mount a legal challenge, according to their members. Beijing loaned Sri Lanka billions of dollars for infrastructure projects when Rajapaksa was president between 2005-2015, putting the country deep into debt.
"All those who cherish democracy, decency and rule of law, must now rally around and defeat the emerging tyranny".
Rajapakse was yet to address the nation after his disputed elevation, but early Saturday attempted to justify the dissolution.
"A general election will truly establish the will of the people and make way for a stable country", he added. 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 party said in a Twitter message that it will meet the elections commissioner to discuss the constitutionality of Sirisena's move.
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.
However, the Extraordinary Gazette Notification issued by the President says he dissolved the parliament "by virtue of the powers vested in me by paragraph (5) of Article 70 of the Constitution of the Democratic Socialist Republic of Sri Lanka to be read with paragraph (2) (c), of Article 33 of the Constitution of the Democratic Socialist Republic of Sri Lanka and paragraph (2) of the Article 62 of the Constitution of the Democratic Socialist Republic of Sri Lanka and in pursuance of the provisions of Section 10 of the Parliamentary Election Act, No. 01 of 1981". Sirisena said he had to fire Wickremesinghe for mismanaging the economy and because of a Cabinet minister's alleged involvement in a plot to assassinate Sirisena.
Sirisena suspended parliament to give himself more time to engineer defections but failed, according to the opposition.
Sirisena had suspended the House until November 16 which was later advanced by two days following worldwide pressure.
Sirisena was critical of investigations into military personnel accused of human rights violations during Sri Lanka's long civil war against a Tamil separatist group, which ended in 2009.
He accused Wickremesinghe of causing instability by refusing to vacate his official residence.