The government serving under President Idris Deby Itno, who himself came to power in a 1990 coup, said a "small group" was involved while police and opposition sources said one of the detainees was an opposition lawmaker.
A police source added that soldiers and civilians also took part in the alleged plot.
"A small group of ill-intentioned individuals attempted to carry out a destabilisation plot against the institutions of the republic," the government said in a statement yesterday.
"This small group... Had been conspiring for more than four months to jeopardise the country's hard-won peace," it said.
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Chad was wracked by civil war between 2005 and 2010.
A police source said that "several civilians and soldiers, including Saleh Makki," an opposition lawmaker, had been arrested Monday.
Opposition leader Saleh Kebzabo said in an email to AFP that he received reports of "numerous" arrests in the capital N'Djamena and that "several sources" had confirmed to him that Makki was among those arrested.
In March 2006, the government said soldiers and police had thwarted a bid to overthrow Deby's regime and kill him by shooting down his aircraft.
The regime has also faced rebellions in the last few years, especially in the east of the country, which borders the Sudanese region of Darfur.
In 2008 rebels from the border region came close to overthrowing the Deby regime, briefly entering the capital before being pushed back with the help of French forces.
Deby himself first came to power in a coup in December 1990 when he overthrew Hissen Habre, who seized power in 1982 after a two-year civil war. Deby had been Habre's military adviser.