Ranil Wickremesinghe said President Chandrika Kumaratunga's surprise action on Tuesday was calculated to sabotage the peace process with Tamil Tiger rebels.
Speaking in Washington, he said: "I pledge to you that your government will not allow this desperate and irresponsible attempt to undermine the peace process... to succeed."
Kumaratunga's move is being seen as a prelude to snap elections and a blow to the country's peace process.
The president, whose party is in parliamentary opposition, dismissed three ministers including Defence Minister Tilak Marapana, whom she had earlier accused of granting too many concessions to the Tamil rebels.
Western and Asian diplomats said they believed the political crisis between Kumaratunga and the cohabitation government would have a serious impact on Norway's attempts to revive the island's stalled peace process.
A senior government minister has said he expects Kumaratunga to call fresh elections after the suspension of parliament lapses on 19 November.
"As much as the government has few options to check the president, she too can't do much to run an administration like this... The only way out is a snap election."
"I pledge to you that your government will not allow this desperate and irresponsible attempt to undermine the peace process... to succeed"
Sri Lankan prime minister
The minister added the government was ready to face an election as it was confident of improving its majority, and the party would face the vote with Kumaratunga controlling the vital state media, security forces and the police.
A government under Kumaratunga's party was elected in October 2000, but collapsed in a year amid defections. In December 2001, her arch rival Ranil Wickremesinghe became prime minister by winning new parliamentary polls.
Tamil separatist conflict
Wickremesinghe revived the Norwegian-brokered peace process to end the Tamil separatist conflict, which has claimed at least 60,000 lives in more than three decades.
He has received strong international support and is due on Wednesday to meet US President George Bush at the White House.
The Tigers said on Saturday they would end an eight-month boycott of peace talks and unveil their first blueprint for power-sharing.
The proposal would turn Sri Lanka into a de facto federal state with more power for the rebels in the northeast of the island where the Tamil minority is dominant.
But Kumaratunga's party has rejected the rebels' plan as a stepping stone for a separate state.
Kumaratunga, who narrowly survived a Tiger assassination attempt in 1999, has repeatedly criticised the government for its concessions to the rebels.
The president also ordered troops to state presses and the state television and radio stations after dismissing the defence, interior and information ministers.
Government troops are guarding
key official buildings
Moderate Tamils said the political instability in the capital undermined peace attempts as it could strengthen the rebel argument that the majority Sinhalese were not serious about a solution.
A Tamil legislator, who does not support the rebels, said a snap election could see the Tigers entering parliament for the first time.
"This would be an opportunity for the Tigers to contest national elections and earn the legitimacy they have been craving," said the Tamil MP, on condition of anonymity.
Parliamentary elections are not due until December 2007, but Kumaratunga's term as president runs until December 2005.