The dissolution, which is expected to be challenged in court, was revealed in an official gazette notification signed by Sirisena which also set the next sitting of parliament for January 17. It said the names of candidates will be called before November 26, and the election held on January 5.
The move comes after an intense power struggle in the past two weeks which followed Sirisena's sudden sacking of Prime Minister Ranil Wickremesinghe and the appointment of former leader Mahinda Rajapaksa, a pro-China strongman, in his place.
In the parliament meeting earlier scheduled for Wednesday, the UNP was planning to show it had the support of more than 113 members in the 225-seat parliament.
President Sirisena has failed to persuade the parliament to support his ouster of his one-time ally Wickramasinghe.
"The dissolution clearly indicates that Mr Sirisena has grossly misjudged and miscalculated the support that he might or could secure to demonstrate support in the Parliament", Bharath Gopalaswamy, director at US-based analyst group Atlantic Council's South Asia Centre, said.
"At the end of the day, he is a victim of his own homegrown crisis".
Wickremesinghe has insisted his firing is unconstitutional.
Australia's foreign minister expressed concern and said Sirisena's action "undermines Sri Lanka's long democratic tradition and poses a risk to its stability and prosperity". But Wickramaratne said a specific clause in the 19th amendment, added in 2015, stipulates that the president can only dissolve Parliament after 4 1/2 years, or earlier if requested by two-thirds of Parliament.
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"Dissolving parliament at this time is illegal and goes against the constitution", JVP general secretary Tilvin Silva told reporters. "The amendment will have been negated if this absurd argument is allowed to succeed".
He accused Wickremesinghe of causing instability by refusing to vacate his official residence. 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.
On Thursday, Wickremesinghe had thanked his supporters in a Facebook video for not letting Sri Lanka be "plunged into the darkness of dictatorship".
Government spokesperson Mahinda Samarasinghe told CNN, however, that there was no provision in parliamentary rules to have a "floor vote" at such short notice.
Following the Parliament's dissolution on Friday, the United National Party - of which Ranil Wickremesinghe is the leader - vehemently rejected the move, and said that it was illegal.
Before he signed the papers dissolving parliament and calling the election, Sirisena appointed allies of his and of Rajapaksa to cabinet positions.
"We will fight the emerging tyranny of Sirisena". The TNA also said it recognised the democratic spirit of elections but questioned the validity of polls being held in the wake of "unconstitutional" moves.
"The US is deeply concerned by news the Sri Lanka parliament will be dissolved, further deepening the political crisis", the US State Department said in a statement.