Princess Grace of Monaco was removed from life support systems on her family's orders Tuesday night after doctors determined that her brain was dead as a result of two severe strokes, the physicians disclosed today.

Dr. Jean Duplay, a neurologist who treated the princess following the automobile accident Monday in which she was severely injured, said that her husband, Prince Rainier, decided that she should not be kept alive artificially. He said the prince's children concurred.

Dr. Louis Chatelaine, director of the Monaco Hospital, said that the fatal strokes were "of equal intensity." One came just before the accident and was apparently its cause and one came some time later, he told reporters.

By about 6 p.m. Tuesday, 30 hours after the princess' car drove off a hillside road, she was clinically dead, according to the doctors.

It was not until four hours later, after Rainier had been in "consultations with his family," that the life support apparatus to keep her heart beating was stopped.

The doctors' latest statements also make clear that Prince Ranier knew from the outset the seriousness of his wife's condition, despite official announcements that she had suffered only broken limbs. But, palace spokesmen said today, this grim prognosis was not relayed to the family's staff or even relatives.

One staff member, Nadia Lacoste, said she was specifically told it was not necessary to return from a vacation in Germany because of the accident.

Most of the palace staff learned for the first time that Princess Grace had suffered a stroke when they heard it Thursday night on the radio.

The condition of 17-year-old Princess Stephanie was also further clarified today. Doctors said she had a hairline crack in her seventh vertebra and is expected to recover. She is still immobilized at the neck, however, and will not be present at her mother's funeral Saturday.

The new details on the timing and manner of Princess Grace's death came as her family and some celebrated friends gathered here to pay their last respects in an atmosphere that seemed at times more appropriate to a Riviera film festival than a funeral.

NBC said it will carry live television coverage of the funeral rites Saturday from 4:30 a.m. to 7 a.m. EDT. Cable News Network (CNN) will also broadcast live coverage beginning at about 6 a.m. A spokesman for ABC said it will present an hour-long, taped special on the events at 4 p.m. CBS said it planned no special coverage.

Flags in Monte Carlo were at half staff and draped with black ribbons. But the streets were jammed with tourists hoping for a glimpse of someone famous. The Monte Carlo Casino was going full tilt, and such a large contingent of the world press converged on this tiny principality that the Monaco press center locked its doors and refused to let anyone in.

President Reagan's wife, Nancy, arrived tonight to lead an official American delegation that includes Navy Secretary John Lehman, a cousin of the princess, and Pennsylvania Gov. Richard Thornburgh, representing the former Grace Kelly's home state.

Britain's Princess Diana will arrive Saturday morning in time for the ceremony, which begins at 11 a.m. local time (5 a.m. EDT).

Other mourners will include the wife of French President Francois Mitterrand, Danielle, and Foreign Minister Claude Cheysson. Ireland, ancestral home of the Kellys, is sending its chief of state, President Patrick J. Hillery. Also on hand will be a host of Europe's lesser known princes and princesses including those from Spain, Belgium, Holland, Sweden, Luxembourg and Liechtenstein.

The film and entertainment world will be represented by a few greats from Grace Kelly's Hollywood past, such as Cary Grant and producer Sam Spiegel. But many of Princess Grace's friends seemed to be sending their spouses, including Frank Sinatra (who had been expected to come himself) and David Niven.

The largest contingent will consist of relatives from the United States and officials from this tiny realm on the Mediterranean that was Princess Grace's home for 26 years.

The funeral will take place in the spacious Monaco Cathedral, only a few hundred yards from the Grimaldi castle where the princess lived with her husband and their three children. While the congregation waits in the cathedral, a family cortege will escort the casket through the streets for the service.

There is no doubt that for many people who live in this resort town and financial center the death of Princess Grace is a genuine loss.

"Our beloved Princess Grace has been taken to God," said a small handwritten sign in the local patois of French and Italian at the front of the church. "The principality of Monaco is in great sorrow. Dear visitors who are visiting this cathedral, pray for our princess."

Inside the church the cathedral choir and the Monaco Philharmonic were preparing selections from Bach, Haydn, Faure and Samuel Barber that will be featured in the funeral ceremony.

But the scene in Monte Carlo away from the church today was not one of mourning. A community that exists for the pursuit of earthly pleasures and money is not transformed easily into a place of sentiment. Busloads of tourists shuttled through the narrow streets. At beaches and swimming pools, scores took advantage of the warm late-summer sun.

But it was the media presence that gave Monaco today its especially gaudy aura. About 600 reporters and technicians are said to be here, and they do not have a great deal to do. Photographers found that the princess' body is no longer lying in state, and funeral guests tended to straggle in rather than arrive in a picturesque cavalcade of limousines.

Similarly for reporters, interest in speculation over any medical secrets the palace may still be keeping, tended to recede with the disclosure Thursday that the princess had had a stroke as she and Stephanie were driving on a steep road in a Rover 3500.

As a result, much of the press seemed to be milling aimlessly with the tourists at key spots such as outside the palace and the swank Hotel de Paris waiting for something to happen.

But their presence for the funeral and plans to televise it live on American television, despite the fact that it will take place before dawn in the United States, guarantee that Princess Grace's death will be, like parts of her life, a media super-event.