Ayatollah Ali Khamenei, the ruling cleric, said during a Monday meeting in Tehran with Qatari Emir Hamad bin Khalifa al-Thani that any ruling by the Special Tribunal for Lebanon was void.
"This court is a kangaroo court and every verdict it issues is rejected," he was quoted by Iran's state-funded broadcaster Press TV as saying. "We hope influential sides in Lebanon will act prudently, so the issue will not be turned into a problem."
The STL is expected to announce the findings of its investigation possibly by the end of the year, though early reports suggest the indictment would be sealed.
Hezbollah, Lebanon's Shiite resistance movement, is expected to be named by the STL, though its leaders say they have evidence to suggest Israel had a hand in the assassination.
Western supporters of the government in Beirut are concerned a Hezbollah indictment may be a destabilizing factor in the region.
Hezbollah Secretary-General Hassan Nasrallah said last week during commemorations marking the Shiite holy day of Ashura that his organization was the target of a smear campaign.
"We refuse to be falsely accused and we shall thwart the objectives behind such indictment," he said.