Former Uruguayan President Juan Maria Bordaberry has been arrested in connection with four political killings during military rule in the 1970s.
Mr Bordaberry is not covered by a 1985 amnesty
The former foreign minister, Juan Carlos Blanco, has also been detained.
The two men are accused of involvement in the killing of two congressmen and two left-wing militants in 1976.
Elected in 1971, Mr Bordaberry went on to govern with military leaders, closing congress and banning parties, before being ousted himself in 1976.
As civilians, Mr Bordaberry and Mr Blanco are not protected by an amnesty passed after the end of military rule in 1985.
Mr Bordaberry presented himself to the authorities at the central prison in the Uruguayan capital, Montevideo, on Friday, a day after his judge ordered his arrest.
He and Mr Blanco, also now in custody, are accused of involvement in the abduction and killing of two lawmakers, Zelmar Michelini and Hector Gutierrez, who had fled the military dictatorship, and two rebels, William Whitelaw and Rosario Barredo.
The politicians were kidnapped from their homes in the Argentine capital, Buenos Aires in May 1976.
Their bullet-riddled bodies were found several days later along with those of the two guerrillas.
Some 180 Uruguayans were killed during military rule, many of them in neighbouring Argentina.
Human rights groups say the killings were the result of secret co-operation between the military governments in power at the time in Uruguay and Argentina.
The current left-wing government of President Tabare Vazquez has made the investigation of human rights abuses committed by the Uruguayan military a major priority.