Ayatollah Ruhollah Khomeini is returning to a truly spartan lifestyle here.

The 78-year old symbol of opposition to the shah's costly Westernization program will move into a bare, undecorated office without table or chairs. The only furnishings are three Persian carpets and long green curtains.

But there are concessions to Western technology -- telephones and television lights.

For several days a school for Islamic girls near the parliament buildings here will be home and office for the ayatollah, and his followers are already crowded outside.

Classrooms have been converted into bureaus dealing with finance, coordination, propaganda, contact with provinces, contact with abroad, and technical and medical centers.

Khomeini's office is on the second floor. In the basement is a makeshift mosque from where his Friday prayers will be relayed to the nation. Photo laboratories and closed-circuit television equipment are on the first floor.

Turbaned mullahs having hurried corridor consultations and workmen making last-minute preparations gave a bizarre atmosphere to this school building today. He is to return in an Air France Boeing 747 jetliner chartered at a cost of $130,000.

Near Paris, a modest supurban villa -- Khomeini's headquarters for the past four months -- buzzed with activity all day today.

His supporters were in a buoyant mood, exchanging telephone numbers and wisecracks with reporters bound for Tehran.

Asked if he was worried about security in a capital wracked by rioting and with pro-shah military units out in force, one close aide said, "ayatollah says Allah will protect us, and I believe him."

Khomeini went into seclusion tonight in the last hours before his return to Iran from 14 years in exile, out-wardly unruffled by threats that he will never reach his homeland alive.

French police have guarded Khomeini for weeks, but they tripled security today. Two platoons of plainclothesmen joined the usual 40-man squad of uniformed gendarmes.

The Paris bureaus of United Press International and Associated Press both received telephone calls warning them to keep their personnel off Khomeini's flight.

UPI bureau secretary Claudine Reix said an apparently recorded voice in heavily accented English said, "Everything will be done to stop Khomeini's flight. The resulting deaths will be his responsibility."

The caller then hung up.

The makeup of the contingent accompanying Khomeini was uncertain until the last minute.

Air France, supplied the chartered Boeing 747 jet for the 6 1/2-hour flight to Tehran, said Khomeini's aides decided how to apportion the 200 available seats among the ayatollah's entourage and journalists.

"There is a big problem," an airline official said. "They wanted to take too many people and not everyone who wanted to go got on the plane."

The 200-passenger limit -- a 747 could carry more than twice that many people -- was a security measure, a Khomeini aide said, to ensure that the craft would be able to return to Paris if it could not land in Tehran.