"God forbid, if the roof collapses, it collapses on all of us," Nasrallah told at least 100,000 Shia Muslims gathered for Ashura, the most solemn event in their calendar.
"Today we are responsible for a nation that came out of the civil war ... but we face acute problems, especially this year and in the past few months," the black-turbaned cleric said.
"As Lebanese, we have no choice for remedying our crises and problems except to discuss and meet, even if we are angry and tense," he said. "We must not repeat the mistakes of the past."
Hizbollah, backed by Syria and Iran, is now a formidable Lebanese political party as well as an anti-Israel guerrilla force that still controls much of south Lebanon since helping end a 22-year Israeli occupation in May 2000.
The death of Hariri, a wealthy Sunni Muslim businessman, sparked an outpouring of public grief mixed with anger against Syria, instinctively held responsible by many Lebanese.
The anti-Syrian sentiments now uniting many of Lebanon's Christians, Druze and Sunnis have not been voiced by Shia leaders counted among the most loyal allies of Damascus. Shias form the country's biggest religious community.
Damascus, which denies involvement in Hariri's killing, echoed Nasrallah's warning. Its allies have often justified Syria's troop presence as a barrier against renewed civil war.
"We agree on how critical the situation is in Lebanon and that it must not be used to stir civil strife," Syrian Foreign Minister Farouq al-Shara said in Damascus.
Hizbollah, the only militia to retain its guns openly since the civil war ended, could come under intense pressure to disarm, in line with UN demands, if Syria left.
Nasrallah, who visited Hariri's family on Friday night to give condolences, called for a speedy investigation into his killing but rejected international involvement in Lebanon.
The United States, which this week recalled its ambassador from Damascus in response to the bombing, told Syria on Friday to co-operate in the investigation or face further sanctions.
Washington imposed some economic sanctions in May, including a ban on US exports to Syria other than food and medicine.
US Secretary of State Condoleezza Rice said Syria could avoid further punishment by changing its policies. "We are not trying to isolate Syria, what we are trying to do is to get Syria to engage in more responsible behaviour," she said.
Washington has previously told Syria to withdraw its 14,000 troops from its tiny neighbour, crack down on Iraqi insurgents and stop backing Hizbollah and Palestinian militant groups.
In September, France and the United States engineered a UN resolution demanding Syrian troops leave Lebanon. Resolution 1559 squeaked through with the minimum number of votes needed.
And the Bush administration wants Security Council members to consider measures that could be taken against Hariri's killers but it was unclear how many council members would agree.
The Beirut government has rejected US and French calls for an international inquiry into Hariri's killing, but the United Nations has asked an Irish policeman to lead a UN team to report on its "circumstances, causes and consequences".