Hourmadji Moussa Doumgor, the Communications Minister, told a press briefing on Wednesday that "a coup attempt was foiled between the afternoon of Tuesday 14 March and today" when Idriss Deby, the President, had been due home from a summit in Equatorial Guinea.
The "masterminds" of the plot were the twins Tom and Timane Erdimi, nephews of Deby and former chiefs of his civilian personal staff, who joined the exiled opposition in December, and renegade former general Seby Aguid, who defected to a rebel movement in February, Doumgor said.
Prosecutors and security forces have opened a probe to track down all those involved and determine their roles in the coup bid.
The army had been tipped off, Doumgor said.
Security forces were out in strength on Wednesday morning in an otherwise quiet Ndjamena, mobile phone links were cut, but nobody reported any gunfire during the night, an AFP journalist said.
"Those who escaped fled in seven vehicles, of which two were intercepted and the occupants neutralised," Doumgor said, without being more specific about their fate.
The aim of the plotters was to detabilise the regime and then assassinate Deby by shooting down his plane when he returned from a six-nation summit of the Central African Economic and Monetary Community (CEMAC).
However, on being told of the plot, Deby cut short his trip to Equatorial Guinea and "returned to Ndjamena last (Tuesday) night and personally took charge of things to restore order," the official spokesman said.
Security forces are out in
Deby, a former guerrilla leader, himself first came to power in a coup in December 1990. Six years later he won the sub-Saharan nation's first multi-party presidential election since independence from France in 1960.
He faces a rebel insurgency mounted by a movement in the east on the border with Sudan, the region from which he launched his power grab. His rebel foes include military men mainly from his own ethnic group, the Zaghawa.
They have formed a Foundation for Change, National Unity and Democracy (SCUD) to overthrow him.
On 10 March, Deby cashiered 70 defectors, including General Aguid and other senior officers, in a decree that booted them out of the army and cancelled pay and their pension and bonus rights.
Aguid and a fellow general had been sent in February to bring about the surrender of the rebels, but instead defected, according to the government.
Deby vowed during a re-election campaign in 2001 to stand down at the end of his second term. But in the first week of March a congress of his Patriotic Movement of Salvation (MPS) chose him as its candidate for a third term and he said he was happy to stand in an election due next May.