Sri Lanka's prime minister has accused the president of trying to create anarchy and chaos in the country.
Troops have been deployed at the state printing press
PM Ranil Wickramasinghe hit out at President Chandrika Kumaratunga after she suspended parliament and sacked three of his key ministers.
She also ordered troops to guard vital buildings, including the state TV.
The two political rivals are at odds over the state of the peace process with the Tamil Tiger rebels.
The rebels have so far only said they are monitoring the situation, but a pro-Tamil Tigers web site, Tamilnet, says the president's actions have "dimmed" the prospects for peace.
Mr Wickramasinghe issued his statement in Washington where he is to brief President George W Bush on negotiations with the rebels.
Mrs Kumaratunga, who has taken control of the ministries, had accused the prime minister of giving too much ground to the Tamil Tigers.
She has wide powers to dismiss Mr Wickramasinghe, 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 latest developments until full details had been received.
Norwegian mediator Erik Solheim, whose country has been closely involved in the peace efforts, told the BBC that they were watching the situation. .
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".
Wickramasinghe and Kumaratunga have an uneasy relationship
Police throughout the country have been put on maximum alert and all leave cancelled.
Mrs Kumaratunga said 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.
Later, in a televised speech to the nation, Mrs Kumaratunga appealed for calm and said she was willing to discuss a "just and balanced" solution with the Tamil Tigers.
The president has also suspended 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 discussed 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.
The BBC's Frances Harrison in Colombo says events have been moving at an extraordinary pace and there is little doubt that the co-habitation in government between the two rivals is unravelling.
Mrs Kumaratunga has been at odds with Defence Minister Talk Mariposa, 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.