JEDDAH, SAUDI ARABIA, AUG. 25 -- Prince Nayef, the Saudi interior minister, charged today that Iran fomented last month's bloody rioting at Mecca to destabilize Saudi Arabia and make the kingdom appear unfit to rule over Islam's holy shrines.

Nayef made his charges at a news conference in which Saudi authorities presented the most detailed public account so far of their version of the July 31 confrontation that killed more than 400 Moslems, most of them Iranian pilgrims, and has led outraged Iranian authorities to urge the overthrow of King Fahd and the Saudi royal family.

With the pilgrimage officially over, Saudi authorities invited more than 100 reporters from Moslem and non-Moslem countries to Jeddah to hear Nayef's presentation. It was an unusual display of concern for world public opinion by a kingdom that traditionally has made discretion a trademark.

The move was interpreted as a sign that the Mecca incident, and particularly the Iranian accusations surrounding it, have deeply affected Saudi rulers.

Fahd and his family have said the Saudi role as guardian of Islam's most sacred shrines, Mecca and Medina, is among their most important duties. Underlining the priority he accords this facet of his reign, Fahd has ordered Saudis to refer to him with the title "custodian of the two holy mosques."

In addition, Ayatollah Ruhollah Khomeini, the Iranian leader, has cited the Mecca killing as a reason to overthrow the House of Saud. Khomeini said in a televised speech Sunday that Saudi rulers are "imbeciles" and that removing Islam's holy sites from their control should have an even higher priority than defeating Iraq or Israel.

This was taken as an appeal to thousands of Shiite Moslems among the Saudi population to rise up against the government, which is dominated by the Wahhabi sect of Sunni Islam. Violent disturbances broke out in 1979 in eastern Saudi Arabia, where the Shiites are concentrated, after a band of fundamentalist Moslems took over the Great Mosque on grounds that the royal family was allowing the country to be contaminated by western influences.

The growth of Iranian militance, particularly since the confrontation with U.S. naval forces in the Persian Gulf, has generated widespread fears of Iranian-connected subversion in Saudi Arabia and other gulf countries backing Iraq in its seven-year-old war with Tehran. Such incidents already have been reported in the last several years in Saudi Arabia, Kuwait and Bahrain.

On Aug. 15, a fire damaged a major Saudi natural gas liquefaction facility at Juaymah in the east, raising questions abroad of possible sabotage. The official Saudi Press Agency, however, said the fire appeared to have been caused by an electrical power surge. Iran has not claimed responsibility for the fire, but its parliamentary speaker, Ali Akbar Hashemi Rafsanjani, has said Iran's leadership considered it "an invisible aid."

{Iranian Prime Minister Mir Hossein Mousavi said in a news conference Tuesday in Tehran that "relations between Iran and Saudi Arabia have been seriously damaged" by the violence in Mecca, Reuter reported.

{Mousavi said, "Iran will use various means to follow up this matter and the outcome of decisions we will make will be felt gradually in the region and the world."}

Nayef, Fahd's brother, charged that the Iranian demonstrators at Mecca were part of a "plot" organized by Khomeini's government to discredit Saudi control over the holy sites. Iranian authorities sought to have the demonstrators penetrate the Great Mosque of Mecca and provoke violence there among masses of praying Moslems, "leading to the deaths of tens of thousands of pilgrims," Nayef said.

"That was the real conspiracy hatched by the leaders of Iran to make it appear Saudi Arabia is not able to provide adequate security," he added.

For that reason, Saudi security forces had orders to halt the march of Iranian pilgrims as it approached the Great Mosque about 6 p.m. on July 31, Nayef explained. Despite an earlier agreement that the demonstration would end at an agreed spot and hour, he added, the marchers sought to continue toward the mosque, forcing Saudi security forces to disperse them with staves.

"What mattered was to prevent them from entering the haram," the sacred interior of the mosque, he said. "But in their stampede, they crushed many innocent people and they killed several security persons and even other pilgrims."

Iranian authorities, who invited reporters to Tehran 10 days ago to hear Iran's version of what happened, said that at this point Saudi security forces opened fire with pistols and automatic rifles, killing a number of people. Bodies with bullet holes and Iranians suffering from bullet wounds were presented to reporters in Tehran as victims of the shooting.

Diplomatic sources in Tehran, quoting their countries' official reports from Saudi Arabia, have said some Saudi policemen opened fire with rifles and tear-gas launchers. But they said it was impossible to know how many pilgrims were killed or wounded by the gunfire.

Saudi officials have said that 402 persons were killed, including 275 Iranians, 42 other pilgrims and 85 Saudi citizens and security officers. Iranian officials have said that 324 Iranians were killed.

Nayef maintained the Saudi denial that security forces fired on the crowd with bullets. But he did not specifically deny that tear-gas canisters were fired.

He said a large number of pilgrims would have been killed if Saudi police had fired bullets into the crowd, which he estimated at 70,000 and Iranian authorities have estimated at 100,000.

"So the number of dead people proves no bullets were fired, because thousands of people would have died," he added. "So the statements that no bullets were fired were the truth. Nobody opened fire. Let it be known that our objective was to disperse the crowd in an ordinary way. Our soldiers only had sticks with them and they still managed to disperse the crowd."

In a comment that drew applause from Saudi officials present, Nayef also said: "But if there had been any danger of their getting into the haram, endangering pilgrims who were praying there, we would have opened fire."