Lebanese PM resigned in a televised address blaming Iranian interference and alluded to fears of an assassination.
Lebanon’s President Michel Aoun has said nothing justifies the apparent detention of Saad Hariri in Saudi Arabia.
In a statement on Wedneday, Aoun called the incident a Saudi act of aggression.
Hariri, a Sunni Muslim politician and longtime ally of Saudi Arabia, suddenly announced his resignation as Lebanon’s prime minister during a visit to Riyadh last week.
He has been promising to return home soon but Saudi leaders say they fear for his safety if he does.
In an interview on Sunday night with the Future TV station owned by him, Hariri drew attention to the risk of economic sanctions and a threat to the livelihoods of Lebanese expatriate workers in Saudi Arabia and its Gulf allies.
Rai became the first Lebanese public figure to visit Saudi Arabia since Hariri announced his resignation in a televised speech from Riyadh on November 4.
Saad Hariri’s father, Rafik Hariri, was killed in a devastating truck bombing in 2005.
In his statement on Wednesday, Aoun said: “We will not accept him remaining a hostage whose reason for detention we do not know.
“Nothing justifies Hariri’s lack of return for 12 days. We, therefore, consider him detained.”
Aoun also said Lebanon had confirmed that Hariri’s family were under detention in their house in Saudi Arabia and were searched whenever they entered or left it.
Up until now, Lebanese officials had said Hariri was probably under either house arrest or in temporary detention in Riyadh.
Under a political deal reached last year, a coalition government was formed in Lebanon, with Hariri as prime minister and Aoun as president.
Hezbollah also joined the 30-member unity government.
Last week, after a ballistic missile fired by Yemen’s Iran-aligned Shia Houthi rebels was intercepted in Riyadh, Saudi Arabia accused Lebanon of declaring war on the kingdom.
Saudi Arabia and its Arab Gulf allies view Hezbollah as a terrorist organisation because of its role in Arab countries ranging from Syria to Yemen.
In a speech from Beirut on Friday, the leader of Hezbollah said Hariri’s “forced” resignation was unconstitutional because it was done “under pressure”.
Hassan Nasrallah said he was sure Hariri was forced to resign as part of what he called Saudi Arabia’s policy of stoking sectarian tensions in Lebanon.
For its part, Iran said on Wednesday that Saudi Arabia had made “multiple mistaken calculations” in the Middle East.
In Lebanon it had “made a completely childish mistake”, Abbas Araqchi, Iranian deputy foreign minister, told Fars news agency.
In another development, Jean-Yves Le Drian, France’s foreign minister, was due to arrive in Riyadh on Wednesday.
He was expected to hold talks on Lebanon with Saudi Crown Prince Mohammed bin Salman and meet Hariri on Thursday, a French diplomatic source said.
Meanwhile, Edouard Philippe, the former French prime minister, has appealed to Hariri to return to Lebanon.
Lebanon was under French control between the world wars and maintains close relations with the country, and especially with the Hariri family.