Former Sri Lankan president and the man who was sworn in as Prime Minister Mahinda Rajapaksa supported the dissolution, and said, "As leaders, it is our responsibility and obligation to give the people the opportunity to voice their opinions on the future of #SriLanka".
Sirisena signed an official notification dismissing the 225-member assembly with effect from midnight, clearing the way for a snap election almost two years ahead of schedule.
The president later called snap polls on Jan 5.
"He has robbed the people of their rights and the democracy that we have enjoyed", the UNP said.
It was not immediately clear how Sirisena can legally dissolve parliament, though his legal experts have said there are provisions for him to do so.
Amid rumours that Sirisena may seek to delay matters further, the European Union, in a joint statement with Norway and Switzerland, said that parliament should vote "immediately when reconvened".
"This is a gross violation of the constitution", Harsha De Silva, a lawmaker in Wickremesinghe's party, said in reference to the dissolution of parliament.
The dissolution of parliament after failing to secure enough support from opposition parties clears the way for a snap election almost two years ahead of schedule.
President Maithripala Sirisena has decided that there will be no snap elections or a national referendum to end the current political and constitutional crisis in Sri Lanka, according to his close aide.
"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. The report said the notification has been sent to the government printer.
Sirisena's sweeping decisions took place Friday via a decree signed just hours after his Sri Lanka Freedom Party's coalition announced it did not have enough votes to support the leader's preferred candidate for the position of prime minister. At least eight have switched sides, but at least 120 deputies in the 225-seat parliament remain loyal to Wickramasinghe.
The United States voiced concern on Saturday after Sri Lanka's president ordered snap elections, as lawmakers warned that U.S. aid was in question.
But Rajapaksa is short of votes in parliament to return to power as prime minister and the sacked, Western-friendly premier, Ranil Wickremesinghe, has refused to back down.
The power struggle on the island of 21 million people has paralysed much of the administration, according to legislators on both sides of the dispute.