Hassan Nasrallah made a rare live appearance in front of thousands of supporters in the southern suburb of Beirut on the eve of Ashura, one of the most important festivals on the Shiite Muslim calendar.
"Tomorrow, we will renew... our stance beside the oppressed, honourable, slain, and besieged Yemeni people," Nasrallah said, referring to Wednesday's morning march in commemoration of Ashura.
"We say to them: You are not alone. We are all Yemen," he said in comments carried live by the group's television station Al-Manar.
Saturday's strike on a funeral ceremony in Yemen was one of the deadliest attacks since the Saudi-led coalition launched a bombing campaign against Shiite rebels there in March 2015.
Nasrallah accused Saudi warplanes of carrying out the raid, echoing Yemen's Huthi rebels who blamed the Riyadh-led coalition for the strike.
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The head of the powerful Shiite movement was met with chants of "Death to the Saud family" from supporters wearing green in commemoration of Ashura, referring to the Sunni-majority kingdom's ruling family.
For Shiites around the world, Ashura is a symbol of the struggle against oppression as it commemorates the assassination of Imam Hussein, grandson of the Prophet Muhammad, in 680.
"Our region is witnessing escalation and instability, unlike what we had seen in recent months, which had indicated the region was heading towards a political solution," Nasrallah said.
"But in the last few weeks, every optimistic political development that had given a positive impression has collapsed," he said.
Nasrallah, whose group has intervened in neighbouring Syria in support of President Bashar al-Assad, said fighting was escalating across Syria after the collapse of a US-Russia truce deal last month.
More than 300,000 people have been killed since Syria's conflict erupted in March 2011.
The war has drawn in regional and world powers, and several attempts at securing a diplomatic solution have fallen flat.