WASHINGTON: Iran "must choose" whether to open the door to opportunity and prosperity, President Barack Obama said Wednesday in a statement marking 30 years since the storming of the US embassy in Tehran.

"We have heard for thirty years what the Iranian government is against; the question, now, is what kind of future it is for," Obama said.

"It is time for the Iranian government to decide whether it wants to focus on the past, or whether it will make the choices that will open the door to greater opportunity, prosperity and justice for its people."

Radical Islamist students captured the city-center US embassy 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.

In Iran security forces braced for possible street protests against President Mahmoud Ahmadinejad during the traditional anniversary parade.

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!"

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 13th of Aban is... a rendezvous so we can remember anew that among us it is the people who are the leaders."