Three main opposition leaders were acquitted on Thursday, including Muhammad Khuna Walad Haid Allah, a former military leader who overthrew current President Muawya Sid Ahmad Walad Taya two decades ago and lost a presidential race against him in 2003.
In all, 181 people have been on trial since November in this Sahara desert nation for involvement in three separate coup attempts between 2003 and 2004.
Those condemned to life prison terms with hard labour included ex-military commander Salih Walad Haninna, Capt Abd al-Rahman Walad Mini, Cmdr Muhammad Walad Shaikhna and Capt Muhammad Walad Salik.
Haninna, the coups' alleged ringleader, had pleaded guilty at the trial, saying he wanted to overturn two decades of dictatorial and corrupt rule by the West African nation's leaders.
During one hearing in December, Walad Haninna said he wanted to "free the Mauritanian people from the grip of a despot who doesn't respect the laws of the country or international conventions".
President Muawya Ahmad Taya
took power in a 1984 army coup
After presiding judge Muhammad al-Hadi Walad Muhammad read the verdicts, applause rang out in the courtroom. Others, family or friends of the accused, wept.
Prosecutors had requested death penalties for 17 of the accused. Two others acquitted on Thursday were Ahmad Walad Daddah, a two-time presidential candidate who heads the Rally of Democratic Forces party and is considered the most powerful opposition leader in the country, and Shaikh Walad Hurma, president of the Party of Democratic Convergence.
Forty-nine people, mostly low-ranking army officers, were ordered to serve 18-month jail terms. Others received sentences ranging from 1 to 15 years in prison with hard labour.
The suspects went on trial en masse at a heavily guarded military base in the town of Wad Naga, 50km east of the capital, Nouakchott. The government had moved the trial from the capital, citing security fears.
The accused were suspected of mounting three coup attempts between June 2003 and September 2004. Only the 2003 coup made it past the planning stage, sparking brief but deadly street fighting in the capital.