Hassan Nasrallah, the leader of Hezbollah, is to address thousands of opposition protesters calling for the Lebanese government to resign.
Demonstrators have now been on the streets of the capital, Beirut, for seven days and another massive rally has been arranged for Sunday.
The opposition has called for "a historic and decisive" demonstration, according to a statement published in Lebanese newspapers on Thursday.
It also urged the Lebanese to "be ready for other forms and means of peaceful protest" to obtain the fall of the government.
The opposition, which includes Shia and Christian factions, said it hoped Sunday would be "a day in which deaf ears and blind eyes would open by meeting the legitimate demands and replacing monopoly with participation and the one-colour government with a national unity government".
Opposition demands
The government has rejected repeated demands from Hezbollah and its allies for increased representation which would give them an effective veto in the cabinet.
Six opposition ministers resigned from the government last month.
Fuad Siniora, the Lebanese prime minister, has refused to step down and urged Hezbollah and the other opposition parties to return to negotiations.
"However long it takes, the Lebanese will have to sit back down together," he said.
"Our hands are extended. Our government is constitutional and we did not accept the resignation of our colleagues," said Siniora. "We have to find a solution by sitting down together, away from tension and confessional incitement."
Michel Aoun, the Christian opposition leader, has warned that his camp would escalate its street protests if the government failed to accept demands for a national unity cabinet.
"If the prime minister and his camp continue to monopolise power, there will be an escalation of popular pressure," Aoun told AFP news agency. "We will paralyse the government, we will force it to go into a deep coma."
Diplomatic efforts
Arab diplomats, fearing a return to civil war, have been attempting to resolve the crisis.
Saud al-Faisal, the Saudi Arabian foreign minister, warned that the crisis in Beirut could damage "the stability, unity, security and autonomy of its political power", at a meeting of Gulf Arab foreign ministers on Thursday.
A Sudanese envoy, who arrived in Beirut on Thursday for talks with the rival factions, supported the opposition's call for a national unity government.
"In our view, the basis of a solution must be founded on the formation of a national unity government and withdrawal of dialogue from the street to parliament," Mustafa Osman Ismail, a special envoy of Sudanese President Omar al-Bashir, said.
Photo Caption
Hassan Nasrallah, the leader of Hezbollah