The statement by cleric Abbas Vaez-Tabasi coincided with rallies by tens of thousands of government supporters calling for opposition leaders to be punished for fomenting unrest after June's disputed presidential election, state media said.
Those who are behind the current sedition in the country ... are mohareb (enemies of God) and the law is very clear about punishment of a mohareb, the representative of Khamenei, who possesses ultimate authority in Iran, said on state television.
Under Iran's Islamic sharia law the sentence for mohareb is execution.
Vaez-Tabasi's remarks came two days after eight people were killed in anti-government protests sparked by the June poll which was won by hard-liner Mahmoud Ahmadinejad.
Political turmoil has entered a new phase in Iran marked by bloody face-offs and arrests, with security forces calling on authorities to deal firmly with opposition leaders.
The establishment intensified a crackdown on the reform movement on Sunday by rounding up leading moderates to try to end street protests after the deadly weekend clashes erupted during the Shi'ite Muslim religious ritual of Ashura.
At least 20 opposition figures have been arrested since Sunday, including three senior advisers to opposition leader Mirhossein Mousavi, his brother-in-law and a sister of Iranian Nobel Peace Prize winner Shirin Ebadi, opposition websites said.
Ebadi said on French radio France Info that Iranian authorities were trying to silence her by arresting her sister.
This arrest is illegal because my sister is a dentist, she is not in any way active in human rights or politics ... and she didn't participate in any protests, Ebadi said.
She said intelligence officials entered her sister's house on Monday night to arrest her without a warrant, rifled through her belongings and confiscated computers.
After U.S. President Barack Obama's condemnation on Monday of Iran's iron fist of brutality against protesters, French President Nicolas Sarkozy on Tuesday condemned the bloody repression of the protests in Iran.
(France) calls for an end to the violence, the release of all jailed opposition activists and respect for human rights, Sarkozy said in a statement.
The elite Revolutionary Guards accused the foreign media of joining hands with the opposition to harm the Islamic state. The British ambassador to Tehran was summoned by the Iranian government to be accused of interference in state matters.
If Britain does not stop talking nonsense it will get a slap in the mouth, Iranian Foreign Minister Manouchehr Mottaki said. The British government said their envoy would respond robustly to any criticism.
On Tuesday, state TV showed footage of huge pro-government rallies in various cities, with demonstrators carrying pictures of the late founder of the Islamic Revolution, Ayatollah Ruhollah Khomeini, and Supreme Leader Ayatollah Ali Khamenei.
The crowed chanted: The blood in our veins is a gift to our leader Khamenei and Death to hypocrites.
Trying to overthrow the system will reach nowhere ... designers of the unrest will soon pay the cost of their insolence, the Revolutionary Guards said in a statement. The opposition, which has joined hands with the foreign media, is backed by foreign enemies.
WAR OF WORDS
The wife of another opposition leader, Mehdi Karoubi, who was fourth in the June vote, said the establishment was responsible for the safety of her family, the opposition Jaras website said. My family and I do not enjoy any security against the rogue forces' nightly attacks, said Fatemeh Karoubi.
Jaras reported that hardliners attacked offices of moderate cleric Grand Ayatollah Yusef Sanei in various cities.
In a heated war of words, the reformist Islamic Iran's Participation Front said in a statement: The only way out of the current crisis is for the authorities to respect the law and apologize to the nation.
Jaras said fresh clashes took place at a Tehran university and also in the central city of Shiraz between students and security forces. The reports could not be independently verified because of restrictions on foreign media covering protests.
Iranian authorities say eight people were killed in clashes on Sunday when supporters of Mousavi used the Ashura religious festival to stage fresh anti-government rallies.
Authorities blame what they call foreign-backed terrorist groups for the killings, including the death of Mousavi's nephew Ali Habibi Mousavi Khamene.
When the June 12 presidential election returned Ahmadinejad to power by a wide margin, thousands of Iranians took to the streets in the biggest anti-government demonstrations in the 30-year history of the Islamic Republic. Authorities reject opposition accusations of vote fraud.
(Editing by Charles Dick)