SANTIAGO - Chilean officials on Monday arrested six people, including four doctors, for their role in the alleged assassination in 1982 of former president Eduardo Frei Montalva, court sources told AFP.

The former Christian Democratic party leader died during routine surgery and his relatives have long suspected that he was the victim of foul play at the hands of secret police operating under the 1973-1990 military dictatorship of Augusto Pinochet.

The four doctors arrested include two who took part in the surgery and two others who had a role in an initial autopsy of Frei's body.

Two other suspects in the case are accused of having spied on Frei, who at the time was seen as one of Pinochet's main political rivals.

The late leader's son, Eduardo Frei Ruiz-Tagle, is a candidate in next Sunday's presidential elections representing the ruling center-left coalition currently led by incumbent President Michelle Bachelet.

The judge in the case, Alejandro Madrid, said for the first time on Monday that he is inclined to believe that Frei's death was in fact, a calculated act of assassination.

"The death of the former president came about as a result of the gradual introduction into his system of unusual, toxic substances... which broke down his immune system," Judge Madrid declared.

Bachelet on Monday hailed the court's arrest order.

"This shows that in Chile, it may at times take a long time for justice to be achieved, but it does eventually come," said Bachelet, who, together with her parents, suffered torture during the Pinochet regime.

"In this case it has arrived and it is for the good of our society," the Chilean leader said, as her South American nation prepares for December 13 elections. Her term at the helm of Chile's government ends in March 2010.

Bachelet added: "Former president Frei denounced human rights violations, and that is probably the reason for the criminal acts taken against him that cost him his life."

Suspicions that Frei had been murdered surfaced after traces of mustard gas, which is known to accelerate infection, and other chemical agents were found in his body during a forensic examination years after his death.

In 2006 a doctor who operated during the surgery, Augusto Larrain, broke his silence by revealing there had been a "black hand" behind the death of Montalva, and referred to a chemical agent that worsened his condition.

Former doctors Patricio Silva and Pedro Valdivia, with the army and police, respectively, were arrested along with civilian Raul Lillo and Montalva's former chauffeur and confidant Luis Berra. The two doctors who performed the autopsy were identified as Helmar Rosenberg and Sergio Gonzalez.

Pinochet seized power in 1973 in a coup in which more than 3,000 people were killed and he ruled the country for the next 17 years. He died in 2006.