Street protests against President Mahmoud Ahmadinejad are expected on Wednesday as Iran marks the 30th anniversary of the capture of US embassy at a time when it is engaged in high-profile nuclear dialogue backed by Washington.

November 4 has emerged as an anti-US day in Iran, with thousands of Iranians, mostly students, gathering annually outside the US embassy building, dubbed the 'Den of Spies', to shout slogans such as "Death to America!" and "Death to Israel!"

The event marks the capture of the embassy on November 4, 1979 -- just months after the Islamic revolution toppled the US-backed shah -- by radical Islamist students who took American diplomats hostage for 444 days.

Since then, the event which was aimed at condemning US policies towards Iran, has become one of the cornerstones of the Islamic regime.

The embassy compound in central Tehran, now run by Iran's elite Revolutionary Guards, is used as an educational centre with occasional exhibitions highlighting the "crimes" of the United States.

But this year the annual anti-US day could be marked by street protests against Ahmadinejad, whose re-election on June 12 triggered the worst political crisis in the 30-year history of the Islamic republic.

Ahmadinejad's main rivals have rejected what they say is his "fraudulent victory" and their supporters have demonstrated at the slightest opportunity against the hardliner.

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

Several reformist websites are urging opposition supporters to gather on Wednesday and protest "peacefully" and not to resort to "violence even if they are attacked."

Opposition leader Mehdi Karroubi, who has alleged that several protesters were raped and beaten in jails in the immediate aftermath of the June election, is expected to participate in the annual event.

In the unrest that followed the June poll, security forces arrested some 4,000 protesters, reformists and journalists, while dozens were killed in the clashes.

Most of the detainees have been released but around 140 are being prosecuted, including some senior reformist politicians, in what opposition leaders have labelled "show trials."

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

Referring to the Iranian date of the capturing of the US embassy three decades ago, Mousavi said: "The 13th of Aban is a... rendezvous so we would remember anew that among us it is the people who are the leaders."

The authorities have warned of a crackdown if any protests are held on Wednesday.

"The police will act against any illegal gathering on the 13th of Aban," Iran's deputy police chief Ahmad Reza Radan told Mehr news agency.

The anniversary comes at a time when Tehran's arch-foe Washington is backing a sensitive dialogue involving world powers over Iran's controversial nuclear programme.

Already turbulent relations between Washington and Tehran in the wake of the capture of the embassy deteriorated even further during the tenure of former US President George W. Bush, who lumped Iran in his "axis of evil" along with North Korea and Iraq.

Ahmadinejad at the same time, during his first term as president, stepped up Tehran's anti-US tirade and Washington became commonly known among Iranians as the "Great Satan."

The two countries, however, are now engaged in diplomatic initiatives launched by Bush's successor, Barack Obama, aimed at resolving a crisis over Iran's nuclear programme.

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

In return as per a UN-drafted deal, world powers would offer Tehran 20 percent enriched uranium to be used as fuel for an internationally-supervised nuclear reactor in the capital.

Enriched uranium can be used either as fuel for reactors or to make the core of nuclear weapons.

Tehran is still to offer a clear response to the deal amid stiff opposition from its senior officials, but the envoy of its ally Moscow, on Sunday urged Iran to accept the offer.