NICOSIA, Cyprus (AP) _ A former Iranian prime minister ordered his followers in Iran to stop their demonstrations against the regime of Ayatollah Ruhollah Khomeini to avoid being exploited, his headquarters reported Saturday.

Shahpur Bakhtiar, the last prime minister under the late Shah Mohammad Reza Pahlavi, had appealed to his supporters by radio last Wednesday to take to the streets to protest Khomeini's policy of continuing the war with Iraq.

But a statement sent to news agencies from Bakhtiar's office in Paris, where he is living in exile, said he was calling a halt to the demonstrations to prevent exploitation of his movement and would soon announce ''new measures'' to overthrow Khomeini.

Khomeini is the fundamentalist Moslem religious leader who organized the revolt that ousted the shah in January 1979. The shah died in exile in Egypt on July 27, 1980.

Bakhtiar's pro-monarchy National Movement of Iranian Resistance claimed there were mass anti-government demonstrations Friday in Tehran, the capital of Iran, and other Iranian cities. It said they were in response to Bakhtiar's radio appeal for demonstrations to demand an end to the war and the establishment of a ''a modern and nationalist government.''

There were reports from Tehran that thousands of people in cars honked the horns and jammed some of the main streets.

But Iran's official Islamic Republic News Agency, IRNA, said only a few hundred persons put on ''a silly display to show their liking of the defunct shah and the corrupt monarchial ways.''

In other developments, Iran's election plans were announced and Khomeini's regime rebuffed a Saudi Arabian effort to mediate a truce in the war.

An IRNA dispatch, monitored here, quoted Interior Minister Ali Akbar Nateq Nouri as saying that preparations for presidential elections will begin in late July, after Parliament passes an election bill.

IRNA also reported that Foreign Minister Ali Akbar Velayati told the Saudi foreign minister, Saud al-Faisal, that Iran will continue its ''just'' resistance in the war.

The war began in September 1980, when Iraq invaded Iran in a border dispute.

IRNA said the Saudi minister, in Tehran on the latest mediation effort, said his government was willing to work with Iran to solve the region's problems. Velayati then presented a detailed list of Iraq's ''war crimes'' and said Iran's resistance was in the best interest of the region, according to the agency.