Last Argentine dictator jailed for 25 years

* Reynaldo Bignone found guilty of kidnapping, torture

* Former "Dirty War" leader sentenced with six others

BUENOS AIRES, April 20 (Reuters) - The last leader of Argentina's 1976-83 dictatorship was sentenced to 25 years in prison on Tuesday for his involvement in the kidnapping, torture and murder of 56 people in a clandestine concentration camp.

Reynaldo Bignone, 82, was convicted along with six other former military and police officers for ordering beatings and electrocutions of dissidents of the military regime.

Dozens of relatives holding pictures of the dictatorship's victims cheered after a judge read out the ruling in a makeshift courtroom set up in a gymnasium.

"Justice was slow in coming but it has finally arrived," said Estela de Carlotto, head of the human rights group Grandmothers of the Plaza de Mayo.

More than 11,000 people died or disappeared during Argentina's "Dirty War," a systematic crackdown on leftists and other opponents of the military regime. Human rights groups say the number is closer to 30,000.

The ruling was the latest by courts that have found new impetus for bringing former dictatorship officials to justice after Argentina's Supreme Court -- at the urging of former President Nestor Kirchner -- struck down two amnesty laws in 2005 that shielded them from charges of human rights abuses.

In its ruling, the court ordered Bignone to serve out his sentence in an ordinary jail. But lawyers for the former leader, who is now under house arrest, are likely to appeal that he not be held in jail due to health reasons.

Bignone was the last of four military de facto presidents in Argentina under the dictatorship, which ended in 1983 amid a deep economic crisis and a humiliating defeat in the war against Britain over the Falkland Islands.

Many of the junta's top leaders are under house arrest on charges of kidnapping babies born to mothers held in captivity during military rule.

Reporting by Walter Bianchi; Writing by Kevin Gray, editing by Philip Barbara