The move comes as part of a major confrontation with her coalition government over the peace process.
Kumaratunga dismissed the defence, interior and information ministers on Tuesday while her political rival Prime Minister Ranil Wickremesinghe was in Washington for a meeting later this week with US President George Bush.
The Sri Lankan president ordered troops to be deployed around key installations, setting off a political crisis after she rejected a landmark peace proposal with Tamil rebels.
Kumaratunga also suspended parliament for two weeks in a move seen by analysts as an attempt to head off an early attempt to challenge the sackings on the floor of the house.
In a one-page statement, Kumaratunga said she sacked the three ministers in the “national interest” but did not elaborate.
“This step has been taken after careful consideration, in order to prevent further deterioration of the security situation in the country,” she said. "The reasons for this action will be made known to the public in due course.”
The president's party has been in opposition in parliament since 2001 but Kumaratunga, who was elected separately, retains the power to dismiss any of Wickremesinghe's cabinet ministers.
Hundreds of heavily armed soldiers surrounded the state printing presses while troops were ordered to the state radio and television stations, said officials.
The state presses print the government gazette that would formalise the ministers' dismissal.
Kumaratunga's Sri Lanka Freedom Party earlier on Tuesday rejected a Tamil Tiger proposal for a de facto federal structure for the island to end three decades of civil war, calling it a foundation for partition.
“The proposals clearly affect the sovereignty of the Republic of Sri Lanka and violate its constitution," her party said in a statement, which urged foreign powers not to push an unacceptable solution to the conflict.
Heavy security was used to make
sure offical documents got out
On Saturday the rebels ended an eight-month boycott of peace talks while peace-broker Norway was trying to organise a meeting between the Tigers and the Colombo government.
Kumaratunga's party lost control of parliament in a December 2001 election to the coalition of Wickremesinghe, who re-launched the peace process with the Tigers.
Kumaratunga had been at loggerheads with the three ministers for some time.
She had openly clashed with Defence Minister Tilak Marapana on the handling of the peace process and accused him of making concessions to Tamil Tiger rebels that compromised national security.
Kumaratunga has accused the interior minister, John Amaratunga, who is also the Christian affairs minister, of unleashing police investigations to harass her supporters.
Information Minister Imthiaz Bakeer Markar crossed swords with Kumaratunga when she recently tried to take over functions of another minister.
All three ministers remain in the cabinet as they hold other posts.
It was not clear how Kumaratunga would take over the running of the defence, interior and information ministries.