TEHRAN - Iran marks on Wednesday the 30th anniversary of the seizure of the US embassy with a traditional anti-American demonstration, with security forces bracing for possible street protests against President Mahmoud Ahmadinejad.

The anniversary, which has turned into a cornerstone of the Islamic regime, comes a day after Iran�s supreme leader Ayatollah Ali Khamenei lashed out at Washington over a vital nuclear fuel deal for Tehran.

Thousands of Iranians, mostly students, traditionally gather each year outside the former embassy building, dubbed the �Den of Spies,� to chant the slogans �Death to America!� and �Death to Israel!�

The event marks the capture by radical Islamist students of the city-centre compound on November 4, 1979 � just months after the Islamic revolution toppled the US-backed shah.

The students, who took 52 American diplomats hostage and held them for 444 days, said they were responding to Washington�s refusal to hand over the deposed shah, Mohammad Reza Pahlavi.

This year, authorities have gone on the alert to ensure that the event is not marred by protests against Ahmadinejad, whose June 12 re-election triggered the worst political crisis in the 30-year history of the Islamic republic.

�Police will confront decisively any gathering which may be formed other than the one opposite the US embassy,� warned Ali Reza Alipour, chief of Tehran�s security police.

Ahmadinejad�s main rivals have rejected what they say is his �fraudulent victory,� and their supporters have demonstrated against the hardliner at each and every opportunity.

On September 18, opposition supporters turned an annual pro-Palestinian rally into a similar anti-Ahmadinejad protest.

Main opposition leader Mir Hossein Mousavi, in a statement on his website Kaleme.com, has hinted at a possible protest rally on Wednesday.

Referring to the Iranian date of the capture of the US embassy, Mousavi said: �The 13^th of Aban is... a rendezvous so we can remember anew that among us it is the people who are the leaders.�

Over the past three decades, many Iranians who led the storming of the embassy have become severe critics of the regime they helped to establish.

Such leading participants as Massoumeh Ebtekar, Abbas Abdi and Mohsen Mirdamadi have since developed into reformists highly critical of the conservative government of Ahmadinejad.

Mirdamadi, who played a key role in the embassy capture, is now in prison accused of trying to topple the government.

The anniversary comes at a time when Washington is backing a sensitive nuclear fuel deal for Tehran brokered by the UN atomic watchdog.

US-Iranian relations deteriorated even further during the tenure of former US president George W. Bush, who lumped Iran into an �axis of evil� along with North Korea and Saddam Hussein�s Iraq.

During his first term as president, Ahmadinejad stepped up Tehran�s anti-US tirade.

And although Washington has made diplomatic overtures towards Tehran under Bush successor Barack Obama, Khamenei said Iran still distrusts the United States.

�Every time they have a smile on their face, they are hiding a dagger behind their back,� he said.

�They are telling us to negotiate, but alongside the negotiation there is a threat... We do not want any negotiation, the result of which is pre-determined by the United States,� he said.

World powers suspect Iran is enriching uranium to make atomic weapons � a charge denied by Tehran � and want its stock of low-enriched uranium (LEU) to be taken out of the country.

In return, world powers would offer Tehran 20 percent enriched uranium as fuel for an internationally supervised nuclear reactor in the capital.