A Yugoslav court today opened the trial of an 86-year-old Croatian accused of responsibility for the deaths of tens of thousands of civilians during World War II. The accused, Andrija Artukovic, is described as the highest ranking war criminal ever to enter the United States.

Artukovic, a former minister in the Nazi-allied independent state of Croatia, was guided by police officers into the Zagreb district court today to hear an indictment that charged he "committed unscrupulous and monstrous offenses against mankind," including ordering the massacres of Jews, Serbs, gypsies and prisoners of war. The indictment said he denied the charges.

Artukovic, who faces a maximum penalty of death by firing squad, was extradited from the United States in February after living in California for 37 years. He had successfully battled extradition and deportation proceedings against him in the 1950s and 1960s and was turned over to Yugoslav authorities only after a new case was brought in 1984.

His trial is one of the most important war crimes cases in Europe since the Nuremburg trials at the end of World War II, legal officials here said. Klaus Barbie, the last major Nazi official to be arrested, was extradited from Bolivia to France in 1983 but has not yet faced trial.

Legally blind and suffering from arteriosclerosis, the slight, white-haired Artukovic had to be assisted to stand and sit during today's proceedings and fell asleep during the reading of the indictment. His defense lawyers argued that he was senile, that he did not understand where he was and that he was unfit to stand trial.

Milko Gajski, chairman of the five-member judicial tribunal, rejected a defense motion that Artukovic's competence be reexamined. The tribunal also rejected a defense motion to delay the trial on the grounds that Artukovic's three lawyers had been allowed only one unsupervised meeting with him by today.

Artukovic, dressed in a dark coat and pants and a white, open-necked shirt, spoke only when asked to cite his birthplace, mother's maiden name, profession and other biographical details. Asked his nationality, he replied "Croatian" in a strong voice.

For the remainder of the four-hour session, which was broken by one recess, Artukovic sat between two guards, his hands folded in his lap and his head often resting on his cushioned chair. Surrounded on three sides by bullet-proof glass partitions, he faced the elevated stand of the tribunal and a portrait of Tito, the Communist leader who conducted the partisan war against the Nazis and Croatian state and ruled Yugoslavia from 1945 until 1980.

Today's session was adjourned just before Artukovic, under Yugoslav legal procedures, would have been asked whether he understood the indictment and if he had any comment on it. In their opening motion, his lawyers said that Artukovic confused the past with the present and sometimes did not even know his own identity.

A representative of a court-appointed team of four neuropsychologists and a cardiologist testified that the group examined Artukovic this morning and found him capable of following the trial. However, tribunal chairman Gajski ordered daily medical bulletins "to study possible mental changes in the defendant."

The trial opening, attended by some 150 Yugoslav journalists and recorded for television and film, was marked by heavy security and a carefully organized publicity effort by Communist authorities determined to reinforce official versions of Yugoslavia's bloody, anarchic warfare between 1941 and 1945.

Following the invasion of Yugoslavia by Germany and Italy in 1941, the country was partitioned and savage fighting began almost immediately involving the occupying forces, their local nationalist allies, and two partisan fronts. Control of most of the modern-day Yugoslav republics of Croatia and Bosnia-Herzegovina was handed over to the fascist Croatian Ustasha organization, which formed a puppet state loyal to the Axis.

As minister of internal affairs and minister of justice and worship in the self-styled independent state of Croatia, Artukovic allegedly supervised concentration camps and police operations in which 200,000 or more persons were killed, according to the indictment. Entire villages of ethnic Serbs were bulldozed and their residents deported or killed. Jews were subject to racial laws and extermination campaigns similar to those in Nazi Germany.

The indictment charges Artukovic with four specific crimes, including two massacres of civilians, the deportation of a prominent ethnic Serbian lawyer who was later killed and the execution of a group of partisan prisoners.

Court officials said the trial is expected to last until the end of this month.