He added that "this despicable crime will increase the Zionists' disasters and strengthen the courage of those who will hit the Zionist regime."
Earlier Wednesday, Israel denied that it was involved in Mugniyah's assassination in Damascus on Tuesday night.
In a letter of condolence to Hizbullah Secretary-General Hassan Nasrallah, the Iranian ambassador wrote, "His pure blood, just like the blood of the other fighters, will claim a heavy price from the enemy.
"I would like to express my condolences following the death of this great commander who had become a shahid (martyr) by the Zionist criminals," Moussavi wrote.
Moussavi took office less than a month ago, after serving as the assistant of Iranian President Mahmoud Ahmadinejad for parliamentary and legal affairs. He is considered one of the Iranian president's associates and advisors.
The Iranian embassy in the Syrian capital is seen as the terror organizations' guidance center and as the place of coordination between the regime in Tehran and Palestinian resistance organizations such as Hamas, the Islamic Jihad and Hizbullah.
Earlier Wednesday, Iran accused Israel of assassinating senior Hizbullah leader Imad Mugniyah and said his death was a result of ''state terrorism by the Zionist regime.''
Iranian Foreign Ministry spokesman Mohammad Ali Hosseini expressed Iran's ''strong condemnation'' of the killing.
''This action is a clear result and example of organized state terrorism by the Zionist regime,'' he said, according to the state news agency IRNA. He called on the world to ''prevent the Zionist regime from taking these actions that are a clear violation of international law.''
Hosseini praised Mugniyah, 45, who was believed to have masterminded a string of attacks in the 1980s and 1990s that killed hundreds of Americans and targeted US, Israeli and Jewish interests in Lebanon and elsewhere. He was also on an FBI wanted list with a $25 million bounty on his head, equal to that the US has put for al-Qaeda leader Osama bin-Laden.
Hosseini called Mugniyah ''a golden page in the history of mankind's fight against the aggressive and occupying Zionists.''
'All of Lebanon will weep'
Tensions were high in Lebanon on Wednesday night ahead of two major rallies scheduled for Thursday. The first an anti-Syrian march marking three years since the assassination of former Lebanese Prime Minister Rafik Hariri, which will be led by his son Saad Hariri.
A short hour later, mass crowds are expected to gather in the Dahiya neighborhood in southern Beirut to pay their respects to Hizbullah operations office Imad Mugniyah, who was assassinated in Damascus on Tuesday.
The polar-opposite gatherings will be taking place against a backdrop of increasing political discord in the country.
Lebanese military forces have already commenced preparations for the deployment of troops throughout the capital city in the hopes of preventing violent clashes. Meanwhile security has been beefed up at checkpoints and barbed wire has been stretched out to separate between the warring camps.
"All of Lebanon will weep for its fallen tomorrow," said anti-Syrian MP Samir Geagea. "We will march to the Martyr's Square (the burial place of Hariri) to say enough! Syria continues to control internal affairs in Lebanon through its allies."
The anti-Syrian camp spoke this week of an expected crowd of at least on million people, although the exacerbated tensions following Mugniyah's assassination may prevent them from reaching that number.
Estimates place the attendance for Hizbullah's march on Thursday at several hundred thousand.
Jumbalatt: There can be no coexistence with Hizbullah
Clashes have also erupted between Hariri's supporters and members of the Shiite Amal movement as well as between those loyal to Druze leader Walid Jumbalatt and opposition leader Wiam Wahab. The Lebanese Defense Ministry outlawed the carrying of firearms until Friday.
Jumbalatt continued to fan the flames in an interview with the Hariri-owned al-Mustaqbal network on Tuesday. "I say this very quietly that there is no way to coexist with Hizbullah," he said, adding that he no longer believed in dialogue with the organization.
"I want a friendly divorce," Jumblatt said. "Let him (Hizbullah Secretary General Hassan Nasrallah) live the way he wants, but we want to live the way we want."
Roee Nahmias, AP and Reuters contributed to this report