"We fear that the Mehlis report could be politicised as this is a report that will have dangerous repercussions since the crime is serious," Secretary-General Shaikh Hassan Nasr Allah told supporters in Beirut on Thursday.
Nasr Allah was speaking as the UN investigator probing al-Hariri's murder, Detlev Mehlis, handed over his report to UN Secretary-General Kofi Annan in New York. The document could be made public on Friday.
"We want to have clear and irrefutable conclusions," said Nasr Allah. "We are in favour of the criminals being punished whatever their identity."
The UN chief said on Wednesday that the Mehlis report would be a purely "technical" one and warned against attempts to politicise it.
Annan also appealed for calm and restraint in anticipation of the inevitable political fallout.
The report's line on Syria's alleged involvement in the Beirut bomb blast that killed al-Hariri will be closely watched worldwide, although Damascus has vehemently maintained it had nothing to do with the 14 February assassination.
Hizb Allah received political and logistical support from Syria during the Israeli occupation of Lebanon and has in subsequent years made no secret of its support for Damascus.