A top Iranian cleric warned on Thursday that continued crackdown of opposition protests could destabilize the government and threaten the very basic of the Islamic republic.

If Iranians cannot talk about their legitimate rights at peaceful gatherings and are instead suppressed, frustrations will build up which could possibly uproot the foundations of the government, no matter how powerful, top cleric Grand Ayatollah Hossein Ali Montazeri said in a statement.

Montazeri, 85-year-old, was once tipped as a possible successor to the father of the revolutionary leader Ayatollah Ruhollah Khomeini.

He also urged the authorities to set up an impartial committee with full power to resolve the worst crisis since the 1979 revolution that ousted the US-backed Shah.

My recommendation to the great and dear Iranian nation is to pursue its logical and fair demands in complete calm, he said.

Turmoil continued in Iran on Thursday. Meanwhile President Mahmoud Ahmadinejad called on U.S. President Barack Obama to apologize for interfering in Iran's affairs.

President Obama said in recent statement that he was appalled and outraged by Iran's post-election crackdown.

Mr. Obama made a mistake to say those things ... our question is why he fell into this trap and said things that previously Bush used to say, the semi-official Fars News Agency quoted Ahmadinejad as saying.

Do you want to speak with this tone? If that is your stance then what is left to talk about ... I hope you avoid interfering in Iran's affairs and express your regret in a way that the Iranian nation is informed of it, Ahmadinejad said.

In recent diplomatic relations, the United States said it would no longer issue invitations for Iranian diplomats to attend July 4 Independence Day parties at U.S. embassies.