The developments were the latest twist in the saga of Hariri, whose fate increasingly resembled a bizarre mystery-thriller that has gripped his nation and sent tensions soaring with Saudi Arabia.
With the Lebanese government in limbo, officials in Beirut said they haven't heard from Hariri since he departed for Saudi Arabia last week. Hariri's own Future Movement party called Thursday for his immediate return home for the "dignity of the nation."
In his pre-recorded resignation speech on Saturday, Hariri accused Iran and its Lebanese proxy, the Shiite militant Hezbollah, of meddling in regional affairs and holding Lebanon hostage. The move shattered his year-old coalition government and stunned the Lebanese, including some of Hariri's aides who had no advance warning that he intended to resign.
Beyond a phone call on Saturday informing President Michael Aoun of his resignation, Hariri has not made contact with Lebanese officials. Aoun has said he would not accept the resignation until Hariri returns to the country and explains the circumstances of his decision to step down.
Late Wednesday night, Hariri's private plane took off from Riyadh and flew back to Beirut - and Lebanese breathlessly awaited his arrival, only to discover he wasn't on board.
The intrigue has thrown the tiny nation in turmoil. It also seemed, ironically, to have united the Lebanese in their resentment of Saudi Arabia's aggressive stance, which many see as an affront.
"How can they hold a prime minister?" asked a Hariri supporter in Beirut who gave only his first name, Abed, saying he feared retaliation for his comments. He said he was at a loss to know what to think, adding that if it turns out that the Saudis were indeed holding Hariri under house arrest "then they have humiliated the whole country by doing this."
On Thursday, Hariri's Future Movement party delivered its sharpest rebuke yet over Hariri's absence, demanding that he be returned home immediately - the clearest sign so far that it believes he is being held by the Saudis against his will.
"The return of the Lebanese prime minister, the national leader, Saad Hariri, and the head of the Future Movement, is necessary to restore dignity and respect to Lebanon at home and abroad," former premier Fouad Saniora said in the statement read on TV.
The Riyadh government, meanwhile, after days of leveling threats against Beirut, ordered all Saudis living in or visiting Lebanon to depart "immediately," and warned against travel to the country.
Hariri, the son of the late Prime Minister Rafik Hariri, who was assassinated in an explosion in Beirut in 2005, is a dual Lebanese-Saudi national with business interests in the Gulf kingdom.
Saudi Arabia sees Hezbollah as a proxy of Iran amid a spiraling rivalry between the two regional Sunni and Shiite heavyweights.
Saudi Minister for Gulf Affairs Thamer al-Sabhan warned earlier this month that his government would deal with Lebanon as a hostile state as long as the militant group Hezbollah was in the Lebanese government. The Lebanese unity government that Hariri formed a year ago includes Hezbollah members - the result of an implicit Saudi-Iranian understanding to sideline Lebanon from the other proxy wars in the region.
That understanding came to an abrupt end with Hariri's resignation, throwing the country back into the forefront of the Sunni-Shiite regional conflicts.
Meanwhile, Hezbollah has found itself in the odd position of coming to Hariri's defense. Hezbollah's leader and one of Lebanon's most powerful figures, Hassan Nasrallah, has speculated openly that Hariri was being held against his will and even said that it appeared Saudi Arabia forced the resignation.
Lebanon is no stranger to intrigue and suspense. Its modern history is full of assassinations, kidnappings and wars that left tens of thousands dead and missing. But the mysterious circumstances surrounding Hariri's resignation have triggered a torrent of rumors - and much trepidation.
A senior Lebanese official said Thursday that Hariri had not been heard from since Saturday. "We don't know anything. All I can confirm is that we have not heard from him since and he has not returned," said the official, speaking on condition of anonymity in line with regulations.
Aides to Hariri in Saudi Arabia, meanwhile, said his decision to remain in the kingdom was due to intelligence reports that he was a target for assassination. The aides, who spoke on condition of anonymity to discuss the sensitive matter, said Hariri was also demanding that Hezbollah stop trying to impose Iran's agenda on the Lebanese government.
Adding to the speculation, the Lebanese daily Al-Akhbar, known for its anti-Saudi stance, reported Thursday that the kingdom had decided to replace Hariri with his older brother, Bahaa Hariri, as its new man in Lebanon. It added that Saad Hariri agreed to pay allegiance to his brother as long as he is set free and allowed to move to Europe and leave politics.
Interior Minister Nohad Machnouk, a member of Hariri's Future Movement and a close aide, dismissed the reports. "We are not a herd of sheep or a piece of property to hand over from one person to the other," he said.
French President Emmanuel Macron, who is currently visiting the United Arab Emirates, declined to discuss Hariri's resignation, saying only that he hopes Lebanon will have a new government soon. He was set to travel to Saudi Arabia later Thursday to discuss the situation in Lebanon and other issues.
"We've had some contact but there's no reason to say anything officially regarding this," Macron said when asked if he had contact with Hariri.
"So far, all I can say is we did not receive any requests" from Hariri for asylum, the French leader said.
Associated Press writers Philip Issa in Beirut, Abdullah al-Shihri in Riyadh, Saudi Arabia, and Jon Gambrell in Dubai, the United Arab Emirates, contributed to this report.
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