Chadian troops have foiled an attempt to oust President Idriss Deby, Chad's communications minister has said.
The Chad rebels want to oust Idriss Deby's government
Hourmadji Moussa Doumgor said a plan to shoot down Mr Deby's plane on his return from abroad had been discovered.
Two senior army officers have been arrested and the coup bid blamed on the president's twin nephews and a general who defected to rebels in the east.
But a key rebel leader denies it was a coup attempt saying rebels had planned a "mass desertion" of soldiers.
A large number of army officers have deserted to join a coalition of rebel groups called the United Front for Democratic Change (FUCD), led by Mahamat Nour from bases in Darfur on Sudan's border with Chad.
Analyst Andrew Manley has told the BBC that in recent months President Deby has been looking increasingly vulnerable, faced with the growing rebellion in the east and a loss of support among neighbouring countries and traditional allies like France.
Mr Doumgor said forces marching towards the capital, N'Djamena, were intercepted in the early hours of Wednesday morning, after the plan was discovered.
Many soldiers have deserted to rebel ranks
He said they fled on seven vehicles, two of which were "neutralised" and Col Eggrey Mahamat and Commander Ali Anour were arrested.
The masterminds of the plot - General Sedi Aguid and the Erdimi twins, all of whom have joined the eastern rebellion - were not amongst those arrested.
However, Yaya Dillo, part of the FUCD coalition, told the BBC's French service that it had been an operation to allow army officers to desert to rebel ranks.
Before the desertion of the president's nephew's last year, Tom Erdimi was in charge of the national oil project and Timane Erdimi headed the cotton industry.
The BBC's Stephanie Hancock says there is a heavy presence of troops in the capital, which is calm.
Our correspondent says people have been at work as usual, but had been wondering about what was going on as the mobile phones have been down for more than 12 hours.
Landlines are still operational, she says.
President Deby was attending a two-day summit of the Central African Economic and Monetary Union (Cemac) in Equatorial Guinea, when he left hurriedly on Tuesday night after the first day's session.
The capital is calmer than it was in December after Chad declared a state of war with Sudan following a deadly attack launched from Darfur by Chadian rebels, our reporter says.
Sudan repeatedly denied allegations made by Chad that it was backing the rebels and sending Arab militias in support.
In February, Chad and Sudan signed an accord to resolve their differences over fighting along the border. Mr Deby seized power in 1990 after launching a rebellion from bases in Darfur.