Soldiers have seized one of the palaces of embattled Madagascar President Marc Ravalomanana in the centre of the capital Antananarivo.
Explosions and gunfire erupted as tanks smashed the palace gates. The central bank was also reportedly taken.
Mr Ravalomanana was in another residence on the outskirts of the city.
Earlier opposition leader Andry Rajoelina called for the arrest of the president and won public backing from the armed forces' self-declared head.
The African Union has condemned the "attempted coup d'etat" and called on Madagascar to respect its constitution.
Soldiers loyal to Malagasy opposition leader Andry Rajoelina try to break down a door as they take over presidential offices in Antananarivo, 16 March 2009
Dozens of soldiers broke into the presidential building
Mr Rajoelina told the BBC he wanted a transitional government that would organise elections in the next 18 to 24 months "at the very latest".
"It's not a military coup," he said. "But the life of the country doesn't stop. You can't have a vacuum of power."
The fierce power struggle on the Indian Ocean island has triggered a military mutiny, looting and violent protests that have left at least 100 people dead since January.
Col Andre Ndriarijaona, who last week said he had replaced the military chief of staff, told AFP news agency that soldiers had seized the presidency "to hasten Ravalomanana's departure".
An unnamed colonel inside the captured building told Reuters: "The palace is occupied. This was our mission for today. For now, we have no more orders."
Mr Ravalomanana was hunkered down about 15km (nine miles) from the city centre at the Iavoloha palace, where hundreds of his supporters were camped out.
"We are against any bloodletting, so we won't go there until we obtain guarantees on the presidential guard's intentions," said Col Ndriarijaona.
The BBC's Jonah Fisher in Antananarivo says round after round of gunfire - believed to be soldiers celebrating - lit up the night sky but the capital is now quiet, except for the howling of stray dogs.
Our correspondent says it may now be a question of when, not if, the president finds himself under siege.
Col Ndriarijaona said the army was virtually all behind the opposition.
"We are there for the Malagasy people. If Andry Rajoelina can resolve the problem, we are behind him," he said. "I would say 99% of the forces are behind him."
Earlier on Monday, the embattled president again proposed a referendum on whether he should complete his term, as a way of resolving the seven-week political crisis.
Mr Ravalomanana, who was re-elected for a second term in office in 2006, has previously said he wants to remain in office until his mandate expires in 2011.
'Thirsty for change'
But the opposition leader rejected Mr Ravalomanana's plebiscite plan and called for the security forces to arrest him.
Mr Rajoelina said: "The people are thirsty for change and that's why we won't have a referendum and will put our transitional government in place."
Our correspondent says Mr Rajoelina has wrapped himself in the cloak of democracy, but he wants to replace an elected head of state without going to a ballot.
The opposition leader, a 34-year-old former disc jockey, says the president is a tyrant who misspends public money.
But Mr Ravalomanana's supporters say his rival is a young troublemaker who has not offered any policy alternatives.
The army has traditionally remained neutral during periods of political volatility since independence from France in 1960.
Under President Ravalomanana, Madagascar's economy has opened up to foreign investment, particularly in the mining sector.
But 70% of the 20 million population still lives on less than $2 (Â£1.40) a day and correspondents say the opposition has tapped into popular frustration at the failure of this new wealth to trickle down.
Course not mate, just an armed insurrection by the military.