Sri Lankan President Maithripala Sirisena dissolved the Parliament late on Friday amid a political crisis in the country. The election will be held January 5 and the new parliament would be convened on January 17.
The US Bureau of South and Central Asian Affairs said in a tweet that Washington was "deeply concerned" by news that the parliament had been dissolved, "further deepening the political crisis".
Wickremesinghe has refused to vacate the official prime minister's residence saying he is the prime minister and had a parliamentary majority.
Wickremesinghe's camp is likely to contest Sirisena's move because of constitutional provisions stating a Parliament can't be dissolved until four and a half years after its election.
"At the moment we have 104 or 105 MPs", UPFA spokesman Keheliya Rambukwella told reporters, adding that the Sirisena-Rajapakse group hoped to secure support from "crossover" legislators.
On Sunday, the president called for parliament to reconvene November 14 amid increasing worldwide pressure to resolve the crisis.
Tensions had been building between Mr Sirisena and Mr Wickremesinghe for some time, as the president did not approve of economic reforms introduced by the prime minister.
Mangala Samaraweera, finance minister in Wickremesinghe's sacked cabinet, said Saturday that their United National Party (UNP) would file a challenge with Sri Lanka's top court next week, saying the president had "kicked the constitution in the teeth".
"Dissolving parliament at this time is illegal and goes against the constitution", JVP general secretary Tilvin Silva told reporters. Before signing the order, Sirisena inducted more ministers into the cabinet headed by Rajapakse.
On Thursday, Wickremesinghe had thanked his supporters in a Facebook video for not letting Sri Lanka be "plunged into the darkness of dictatorship". Beijing loaned Sri Lanka billions of dollars for infrastructure projects when Rajapaksa was president between 2005-2015, putting the country deep into debt.
In what opponents condemned as an illegal move, Sirisena dissolved parliament on Friday and announced a January election after it became clear his designated Prime Minister - former president Mahinda Rajapaksa - would not be able to command a majority in the current legislature. "We will fight this dictator to the end". The party vowed to discuss the situation with the country's election commissioner.
Mark Field, the British minister of State for Asia and the Pacific, said: "As a friend of Sri Lanka, the United Kingdom calls on all parties to uphold the constitution and respect democratic institutions and processes".
Urging the Indian government to condemn the "murder of democracy", the Dravida Munnetra Kazhagam (DMK) leader said the Union government must take steps to ensure a dignified life for Tamils with peace and security.
Rajapaksa indicated what was coming hours before the dissolution in a speech.
The political crisis was started when he unconstitutionally ousted the Prime Minister, Ranil Wickremesinghe on October 26 and appointed former president Mahinda Rajapaksa as the Prime Minister of the United People's Freedom Alliance (UPFA) government.