"We won't allow the resistance to be degraded. It is the most precious thing we have," Nasrallah said in a televised speech.
The Hezbollah leader said the investigation must be based in Lebanon in order to be fair, and called it "an Israeli project".
He said the public should mistrust a tribunal "made up of Americans and the British government, and investigation officers who come from intelligence (agencies) with strong ties to the Israeli Mossad".
"Will the path on which this tribunal is walking reach truth and justice?" Nasrallah wondered on the air.
"Anything that is not based on truth will be like another assassination of Rafik al-Hariri because it will hide the murderers from view while punishing the innocent," he said, adding that Hezbollah "has evidence that will speak out in good time".
Nasrallah claimed the investigators had presupposed Hezbollah members were guilty. "We demand justice that will punish those who murdered al-Hariri and anything other than that will be injustice," he said, and called for an inquiry committee based in Lebanon.
On Thursday Nasrallah carried a similar speech at a press conference in which he expressed concern that Hezbollah would be blamed for the assassination rather than Syria.
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