He said: "Mr prime minister go home, it's better for you because you cannot captain the ship."
He also repeated opposition accusations that Siniora was a subordinate of the US.
The ruling coalition is made up of Sunni Muslim, Druze and several Christian parties, while the opposition consists of Shia groups, Hezbollah and the Christian Free Patriotic Movement party.
Siniora and the government's Sunni-led majority have refused the oppositions demands and accused Hezbollah of trying to stage a coup in the wake of their 34-day war against Israel in July and August.
On Sunday, the pro-opposition Ad-Diyar newspaper's front page read: "Opposition to rally largest crowd today."
The paper said the opposition would escalate its campaign from Monday, calling for strikes that would lead to a civil disobedience campaign.
"Lebanon is a country of eighteen sects. If everyone is seeking power, the only solution is to have a unity government. Therefore, I would have to agree with Mr Nasrallah and his logical idea."
Biqaa_gharbi, jubjannine, Lebanon
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For his part, Siniora, in speech on Sunday that was telecast by Al Jazeera and other Arabic channels, said: "Our hand is stretched out to all parties ... we will not close the doors. We will keep the doors open for a bright future.
"And as we stood defiant against the Israeli aggression, united steadfast, defending our country, we will stand in the face of this ordeal defending out values, the democratic system and national reconciliation and harmony, while opening out hearts to everyone, based on established national patriotic and democratic principles."
Hashem Ahelbarra, an Al Jazeera correspondent at the protests in Beirut, said: "People here are saying they will stay here for as long as it takes to topple the government."
One Shia protester has been killed and several people hurt in shooting incidents and clashes between supporters of both sides over the past week.
Thousands of Lebanese soldiers and police have been stationed around the the capital ahead of the protest.
Meanwhile, a vast crowd, estimated by organisers at "hundreds of thousands", rallied on Sunday in northern Lebanon in solidarity with the Siniora government.
Waving Lebanese flags and chanting pro-government slogans, the multitude gathered in an open-air exhibition centre in the Sunni Muslim-dominant coastal city of Tripoli.
The rally coincided with one of opposition followers - estimated by an army spokesman also to be in the hundreds of thousands - outside Siniora's offices in downtown Beirut to back demands for his resignation.
"Siniora will not fall, Lahoud will fall," MP Saad Hariri - leader of the parliamentary majority which has demanded the resignation of Emile Lahoud, the pro-Syrian president - said in a speech broadcast live at the Tripoli rally.
Earlier at the rally, Ahmad Fatfat, the youth affairs minister, said: "The government will hold."
"We tell those who are trying to stage a coup d'etat: You will fail and the government will not step down."
A number of pro-government rallies have also been held in several areas of Lebanon.
Sunday will also see a memorial for Gebran Tueni, the Lebanese journalist who called for the withdraw of Syrian troops from Lebanon and was assassinated in 2005.
The memorial will be held at Tueni's newspaper, An-Nahar.