By Hannah Hennessy BBC News, Lima

Relatives of some of the students who died protested at the court

It is the first human rights trial against Vladimiro Montesinos, a close aide of ex-President Alberto Fujimori.

Montesinos is accused of leading the death squad, Colina Group, during a campaign to root out Maoist rebels.

He faces nearly 100 other charges, from corruption to arms trafficking, and is already serving 15 years in jail.

Montesinos sat impassively as he went on trial.

The 60-year-old former intelligence chief faces a possible 35 years in jail if he is convicted of leading Colina Group.

He is accused of ordering members of the group to murder 15 people in a poor area of the Peruvian capital, Lima, in 1991.

Months later, the Colina Group is alleged to have kidnapped nine university students and their professor. Their burned bodies were discovered a year later.

Corruption scandal

Both of the massacres occurred at a time when the government of Alberto Fujimori was trying to root out people who sympathised with the Maoist-inspired Shining Path rebel group.

Montesinos could face 35 years in jail if found guilty

The group waged a bitter insurgency against government troops throughout the 1980s and 1990s, during which tens of thousands of people were killed.

Prosecutors in this latest trial believe the ex-president condoned the two massacres, although he is not one of the 56 other defendants on trial.

Mr Fujimori, who denies any wrongdoing, fled to exile in Japan five years ago when his government collapsed after a massive corruption scandal.

Tokyo will not extradite him to face murder and corruption charges because he has Japanese citizenship.

Montesinos sparked the corruption scandal in 2000, when a video appeared apparently showing him bribing an opposition politician, Luis Alberto Kouri.

He has been in jail since his arrest in 2001, and was given a 15-year sentence last year for embezzlement, conspiracy and corruption.

It was the fifth guilty verdict against him since he went on trial in 2002.