The Mauritanian government has arrested Saleh Ould Hanenna, the mastermind of last year's military uprising against President Maaouiya Ould Taya, who had been on the run for 16 months.
Attorney General Mohamed El Ghaith Ould Oumar said the former army major had been caught on Saturday in Rosso, a town on the southern border with Senegal which is the main crossing point between the two countries.
He told reporters on Sunday that Ould Hanenna, who is also accused of planning two further coup attempts in August and September this year, had been arrested by a joint force of soldiers, police and paramilitary gendarmes.
A police spokesman said three popular Islamic fundamentalist leaders had been arrested for questioning because of their suspected involvement with the captured rebel.
The attorney general said Ould Hanenna, who was drummed out of the army in 2000, would be put on trial for trying to overthrow the government by force of arms and using "criminal gangs" to try and seize control of the army and conspire against the state.
Ould Hanenna was the mastermind of a failed coup attempt on 8 June 2003 which led to two days of heavy fighting in the capital Nouakchott before forces loyal to Ould Taya regained control.
The government subsequently rounded up 131 members of the armed forces who are due to go on trial on 21 November for their part in the coup attempt. However, Ould Hanenna and several other ringleaders escaped capture at the time.
The government subsequently accused the rebels of plotting further coup attempts in August and September this year with the backing of Libya and Burkina Faso.
On Sunday, the police spokesman went further by accusing the rebels of running a training camp in the rebel-held north of Cote d'Ivoire.
Following the failure of last year's attempt to seize power, Ould Hanenna and his comrade in arms Mohamed Ould Cheikhna announced the formation of an armed opposition movement called "The Knights of Change."
This allied itself with radical Islamic groups in Mauritania, which have long been persecuted by Ould Taya's pro-western government.
Ould Taya is a former army colonel who came to power in a 1984 coup. Originally seen as a Baathist with close links to the former Iraqi leader Saddam Hussein, he subsequently developed close ties with France and the United States.
Ould Taya angered many of Mauritania's staunchly Muslim 2.8 million people by establishing diplomatic ties with Israel and over the past two years he has cracked down repeatedly on Islamic fundamentalist groups in the country.
Alarmed by the emergence of armed Islamic fundamentalist groups active across national borders in the Sahara desert, the United States sent a military training team to Mauritania earlier this year to help the country's armed forces set up an anti-terrorism unit.
The police spokesman said one of the Islamic radicals arrested on Sunday was Mohamed El Hacene Ould Dedaw, the head of the Mauritanian Islamic Movement. The day beforehand he had urged Ould Taya to establish a dialogue with the opposition in comments broadcast on the Arab television channel Al Jezira.
The other two Islamic leaders arrested were named as Mohamed Jamil Ould Mensour, vice-president of the Democratic Forum Party, an opposition group which has been denied legal recognition by the government, and Moctar Ould Mohamed Moussa, Secretary General of the National Alliance against Zionist Infiltration and for the Defence of Palestine and Iraq.
Many opposition protest demonstrations in Nouakchott take the superficial form of marches in support of the Palestinian cause in order to avoid being banned by the government.
Mauritania's often divided opposition movement united on Monday to protest at the arrest of the three Islamic activists.
"We express our solidarity with these personalities who are above all suspiscion and have always made clear their strong position on violence vis a vis what is good and useful for the country - that is to say agreement, dialogue and tolerance - and we demand their release," said Massaoud Ould Boulkheir, the head of the Popular Progressive Alliance Party, speaking on behalf of a coalition of nine opposition groups.