Lebanon's President Michel Aoun has for the first time publicly accused Saudi Arabia of detaining its prime minister.
Prime Minister Saad al-Hariri abruptly announced his resignation during a visit to the Saudi capital Riyadh on 4 November.
Mr Aoun said "nothing justified" Mr Hariri's continued absence. It was a breach of his human rights and an "act of aggression" against Lebanon.
The Saudi government has denied holding him against his will or putting pressure on him to resign in an attempt to curb the influence of its regional rival, Iran.
The Shia Islamist Hezbollah movement, an Iranian proxy that Riyadh considers a terrorist group, is part of the unity government that Mr Hariri formed last year.
A French presidential source said Mr Hariri would travel to France "in the coming days." French President Emmanuel Macron invited Mr Hariri and his family to France, after speaking by telephone to the prime minister and Saudi Crown Prince Mohammed bin Salman, the Elysee Palace said in a statement.
On Sunday night, Mr Hariri said in a television interview that he was free to leave Saudi Arabia and that he had left Lebanon in order to protect himself.
President Aoun's official Twitter account said: "Nothing justifies the failure of Prime Minister Hariri to return for 12 days. Therefore, we consider him to be held and detained, in violation of the Vienna Convention and human rights law.
"No decision can be made on a resignation from abroad. He should return to Lebanon to present his resignation or withdraw it, or to discuss the reasons for it and how to address them."
Mr Aoun is a Maronite Christian former army commander who is an ally of the Islamist militia and political party Hezbollah.
He told journalists and media executives Mr Hariri and his family were restricted in freedom of movement and were under surveillance.
Mr Hariri responded swiftly to the president's comments, writing on Twitter: "I want to repeat and affirm that I am perfectly fine and I will return, God willing, to dear Lebanon as I promised you, you'll see."
A member of Mr Hariri's political party, the Future Movement, also told Reuters news agency that the prime minister and his family were not being detained.
Mr Hariri had accused Iran of sowing "discord, devastation and destruction" in the region and said he sensed there was an assassination plot against him.
His father Rafik - himself a former Lebanese prime minister - was killed in a suicide bombing in Beirut in 2005. Several members of Hezbollah are being tried in absentia at a UN-backed tribunal at The Hague in connection with the attack, though the group has denied any involvement.
Mr Hariri, a Sunni Muslim who became prime minister for the second time in late 2016 in a political compromise deal that also saw Mr Aoun elected president, has close ties to Saudi Arabia.
His father made his fortune in the Gulf kingdom and he owns properties there. He also holds both Lebanese and Saudi citizenship, and Riyadh backs his party.