There was no line-up of Saudi princes or ministry officials, as would typically greet a prime minister on an official visit to King Salman, senior sources close to Hariri and top Lebanese political and security officials said. His phone was confiscated, and the next day he was forced to resign as prime minister in a statement broadcast by a Saudi-owned TV channel.
The move thrust Lebanon back to the forefront of a struggle that is reshaping the Middle East, between the Sunni monarchy of Saudi Arabia and Shia Iran.
Their rivalry has fuelled conflicts in Iraq, Syria and Yemen, where they back opposing sides, and now risks destabilising Lebanon, where Saudi has long tried to weaken the Iran-backed Hezbollah group, Lebanon's main political power and part of the ruling coalition.
Sources close to Hariri say Saudi Arabia has concluded that the prime minister - a long-time Saudi ally and son of late Prime Minister Rafik Hariri, who was assassinated in 2005 - had to go because he was unwilling to confront Hezbollah.
Multiple Lebanese sources say Riyadh hopes to replace Saad Hariri with his older brother Bahaa as Lebanon's top Sunni politician. Bahaa is believed to be in Saudi Arabia, and members of the Hariri family have been asked to travel there to pledge allegiance to him, but have refused, the sources say.
"When Hariri's plane landed in Riyadh, he got the message immediately that something was wrong," a Hariri source told Reuters news agency. "There was no one waiting for him."
Saudi Arabia has dismissed suggestions it forced Hariri to resign and says he is a free man. Saudi officials could not immediately be reached for comment on the circumstances of his arrival, whether his phone had been taken, or whether the kingdom was planning to replace him with his brother.
Saad has given no public remarks since he resigned and no indication of when he might return to Lebanon.
Hariri was summoned to the kingdom to meet King Salman in a phone call on Thursday night, November
Before departing, he told his officials they would resume their discussions on Monday. He told his media team he would see them at the weekend in the Red Sea resort of Sharm el-Sheikh, where he was due to meet Egyptian President Abdel Fattah el-Sisi on the sidelines of the World Youth Forum.
Hariri went to his Riyadh home. His family made their fortune in Saudi Arabia and had long had properties there. The source close to Hariri said the Lebanese leader received a call from a Saudi protocol official on Saturday morning, who asked him to attend a meeting with Crown Prince Mohammed bin Salman.
He waited for about four hours before being presented with his resignation speech to read on television, the source said.
"From the moment he arrived they (Saudis) showed no respect for the man," another senior Lebanese political source said.
Hariri frequently visits Saudi Arabia. On a trip a few days earlier, Prince Mohammed bin Salman had arranged for him to see senior intelligence officials and Gulf Affairs Minister Thamer al-Sabhan, the Saudi point man on Lebanon.
Hariri came back from that trip to Beirut "pleased and relaxed", sources in his entourage said. He posted a selfie with Sabhan, both of them smiling. He told aides he had heard "encouraging statements" from the crown prince, including a promise to revive a Saudi aid package for the Lebanese army.
The Hariri sources say Hariri believed he had convinced Saudi officials of the need to maintain an entente with Hezbollah for the sake of Lebanon's stability.
Hezbollah has a heavily armed fighting force, in addition to seats in parliament and government. Saudi-backed efforts to weaken the group in Lebanon a decade ago led to Sunni-Shia clashes and a Hezbollah takeover of Beirut.
"What happened in those meetings, I believe, is that (Hariri) revealed his position on how to deal with Hezbollah in Lebanon: that confrontation would destabilise the country. I think they didn't like what they heard," said one of the sources, who was briefed on the meetings.
The source said Hariri told Sabhan not to "hold us responsible for something that is beyond my control or that of Lebanon". But Hariri underestimated the Saudi position on Hezbollah, the source said.
"For the Saudis it is an existential battle. It's black and white. We in Lebanon are used to gray," the source said.
Sabhan could not immediately be reached for comment.