Sri Lankan President Chandrika Kumaratunga has suspended parliament and sacked three powerful ministers, plunging the country into crisis.
Security forces have been put on high alert
Troops have been ordered to guard key installations, including the state television station.
Prime Minister Ranil Wickramasinghe, her rival, accused the president of seeking "chaos and anarchy".
Mr Wickramasinghe, who has a small majority in parliament, is in the US to brief President Bush on negotiations with the Tamil Tigers.
Mrs Kumaratunga, who has taken control of the ministries, is at odds with the government over the peace process.
She has wide powers to dismiss Mr Wickramasainghe, who defeated her party in December 2001 elections, and call a new poll.
Days ago, Tamil Tiger rebels submitted a power-sharing peace proposal.
Mrs Kumaratunga's Sri Lanka Freedom Party, which is the parliamentary opposition, said in a statement it was concerned about the Tigers' proposals, for a self-governing authority in the north-east of the country.
Tamil Tiger spokesman Daya Master said no comment would be made on the developments in Colombo until full details had been received.
Mr Wickramasinghe said he would be resolute in implementing his mandate and would continue his mission in the US.
He said would not allow Mrs Kumaratunga to "undermine the peace process and economic prosperity".
"The irresponsible and precipitous action of the
president is aimed at plunging the country into chaos and
anarchy," he said in a statement.
Addressing the Sri Lankan people and the security forces, he said: "I call upon all of you ... to remain calm and vigilant in the face of this deliberate attempt to end the peace process."
Military spokesman Colonel Sumedha Perera said "several platoons" of troops had been deployed to the state printing press, TV and radio, and power stations to "prevent any disturbances".
There were reports that hundreds of heavily armed soldiers had surrounded the printing press, where a government gazette is expected to publish news of the dismissals.
Wickramasinghe and Kumaratunga have an uneasy relationship
Police throughout the country have been put on maximum alert and all leave cancelled.
The BBC' Frances Harrison in Colombo says the deployment is fairly routine in a country where security has traditionally been high, but it does seem to indicate some nervousness that this latest move by the president might encounter resistance.
Mrs Kumaratunga said earlier that she had dismissed the information, defence and interior ministers in the "national interest".
"This step has been taken after careful consideration, in order
to prevent further deterioration of the security situation in the
country," she said in a statement, adding that the reasons for this action would be made known in due course.
This measure was followed by the suspension of the parliament until 19 November, a move which analysts say in effect pre-empts any attempt to challenge the sackings.
The suspension means that a reading of the country's budget, which was due to be discusssed on 12 November, will be put on hold.
Mrs Kumaratunga has an uneasy relationship with Mr Wickramasinghe's government, who defeated her party at parliamentary elections in December 2001.
Our correspondent says the sackings seem to have ended any hope of further co-habitation.
Mrs Kumaratunga has been at odds with Defence Minister Tilak Marapuna, Interior Minister John Amaratunga and Information Minister Imthiaz Bakeer Markar over their handling of the peace process.
All three ministers still hold other cabinet-level posts.
The US Government said on Monday it believed it was possible for both sides to reach agreement on the issue of an interim administration.
But Washington has failed to comment on the details of the Tigers' proposal.
The rebels signed a ceasefire with the government in February 2002, bringing to an end two decades of fighting which have left more than 60,000 people dead.
In their latest proposals, the Tigers dropped a demand for independence in favour of regional autonomy.
But opposition politicians say the proposals are in breach of the constitution and the Tamils are using the interim administration to prepare the legal ground for breaking away.