Mauritania, an Arab-dominated nation straddling Arab and black Africa, has moved the trial from the capital, Nouakchott, to a military barracks in the desert 50km to the east.
Anti-aircraft batteries, artillery and thousands of infantry soldiers surrounded defendants in the isolated barracks courtroom on Sunday.
"This will be a parody of justice," defence lawyer Ibrahim Abatti said as defence attorneys complained of what they said were numerous irregularities, including military officers on the jury for what is supposed to be a criminal civilian trial.
A total of 181 officers and other military members and civilians - including former president Muhammad Khuna Walad Haid Allah, and the leaders of two opposition parties - are accused of plotting to overthrow President Muawia Sid Ahmad Walad Taya's 20-year regime.
Courts accuse the defendants of mounting three attempts in 15 months against Taya's government.
The most aggressive attempt, in June 2003, had loyalist and coup forces battling with tanks and guns in the streets of the capital.
Defendants face the death penalty if convicted. The trial is expected to last several months.
All but a few dozen of the hundreds of family members who came for the trial were turned back on Sunday.
Taya, who himself took power in a 1984 coup, has increasingly allied his Muslim republic with the West, and cracked down on opposition figures.
The opposition accuses him of crushing dissent, torture and other rights abuses.