The Inter-American Human Rights Commission, invited by the Colombian government to evaluate fairness of the country's military justice system at the insistence of the M19 guerrillas holding 17 diplomats hostage here, began a week-long investigation today that could effect the outcome of the embassy siege.

The commission, an independent body related to the Organization of American States, will attend at least one session of the controversial mass trials, now under way in the military courts here, of more than 200 leftists charged with political violence and other "subversive" activities.

The government of President Julio Cesar Turbay Ayala, who met with the commission members this morning, is hoping the commission will find that the military courts are impartial and that the alleged terrorists' incarceration and trial are warranted.

The M19 hopes that the commission will report that the military judges are not impartial and that many of those being tried were tortured and arrested for their political beliefs.

Among those on trial are at least seven leading members of the M19.

Members of the Inter-American Human Rights Commission, which is led by Thomas Farer of Rutgers University, said in interviews yesterday that they are keenly aware of the sensitive situation they are in because of the impact their report could have on the outcome of the hostage siege and also because Colombia is the first of the hemisphere's democracies to which the commission has been invited.

Last week Amnesty International issued a highly critical report on the human rights situation here.

Amnesty found that those arrested for alleged political crimes have been "systematically tortured in military installations" and that the government's antiterrorist campaign "has gone beyond the limits of countering violent opposition."

Turbay rejected the London-based human rights group's findings, accusing Amnesty of bias and of failing to present evidence to support its conclusions.

Turbay admitted, however, that there may have been some mistreatment of prisoners in the military jails by "low-level functionaries. . . . But under no circumstances," he said, "is this a general practice."

On Saturday, the M19 had proposed a meeting of Colombia's political and military leaders, including Turbay, to discuss social reforms as a way of ending leftist political violence. Government spokesman German Zea today emphatically rejected the M19 proposal, saying the government would not "sit down with subversives to discuss its programs."