Demonstrators are likely to be chanting 'death to the dictatorship' as well as 'death to America' at this year's rallies

November 4 has been a big day in Iran for the past 30 years — ever since the "students following the imam's line" stormed and seized the American embassy in Tehran in one of the most memorable moments of the Islamic revolution of 1979.

The unremarkable brick building that Tehran taxi drivers still universally identify as the "nest of spies" is the scene of annual mass rallies designed to highlight the regime's enduring hostility to the United States. But today's ritual protests against "global arrogance" could turn into one of the biggest displays of opposition to the Iranian government since President Mahmoud Ahmadinejad claimed victory in last summer's disputed elections.

Mir Hossein Mousavi, the man who says he won in June, plans to exploit the occasion to remind Iranians – and the world – that the flame of democracy has not been extinguished despite a crackdown that has seen about 70 dead, thousands of arrests, show trials, a gagged media, divisions at the top and a profound crisis about the legitimacy of the Islamic system.

November 4, Mousavi told his supporters this week, was a "rendezvous so we would remember anew that among us it is the people who are the leaders".

In response, the Revolutionary Guards have been warning Iranians to "exercise vigilance in regard to the likelihood of mischief and plots by the enemy's agents and some unaware and misguided people".

Other elements of the regime, including the powerful Guardian Council, have reinforced the message that protests are the work of malcontents linked to "foreign oppressors". This is the old insinuation that if it were not for sinister outsiders fomenting trouble, Iranians would be perfectly happy. English-language Press-TV has warned of "foreign media hype".

The scale of these warnings seems to suggest that the authorities are worried.

Mousavi and other leaders of the Green movement hope to repeat a trick they pulled off in mid-September, when their supporters succeeded in partially hijacking the annual al-Qods (Jerusalem) day celebrations, which expresses solidarity with the Palestinians. The logic of these moves is that since the government will never cancel such iconic occasions in the revolutionary calendar they should be exploited to stage demonstrations.

The prospect, then, is of pro-reform protesters festooned in green chanting "Death to dictatorship" instead of the more familiar "Death to America".

Disinformation seems to be playing a part in countermeasures undertaken by the security forces. Reformist websites have called on people to ignore reports of protests planned outside the Russian embassy, ostensibly to protest at Moscow's speedy recognition of Ahmadinejad's disputed "victory", and of plans by the licensed thugs of the Basij militia to storm the British embassy compound in the next street.

Iran-watchers and analysts at home and abroad say today's "nest of spies" events could be a test of how hard the regime is prepared to crack down again after a period of relative calm in recent weeks — and perhaps provide some clues about apparent disagreements between hardliners and more pragmatic elements.

Another point to watch is the coherence of the Greens' leadership: what is the role of Mehdi Karroubi, another defeated reformist candidate, and of Mohammed Khatami, the former president whom Ahmadinejad defeated in 2005?

Zahra Rahnavard, Mousavi's popular wife, struck a defiant tone earlier this week by demanding the release of all women detainees. Students have been organising in force across the country – an ominous reminder to the authorities of the crucial role the universities played in protests back in 1979.

International interest in Iran's internal situation has faded recently – not least because of the expulsion of almost all non-Iranian journalists and severe restrictions imposed on the few who remain. But the US and other western governments will be watching carefully to see if the domestic situation has an impact on the stand-off over Iran's nuclear ambitions – a far higher priority for them than prospects for internal change.

Following the usual script, pro-government media have been accusing Greens of trying to influence the nuclear talks, with heavy hints of treachery and links with the US. Some have spoken out on this issue and other aspects of Iranian foreign policy. But few have gone as far as the exiled film-maker and Mousavi spokesman, Mohsen Makhmalbaf, who has publicly apologised for the takeover of the "nest of spies" and even urged President Barack Obama to designate November 4 as a day of friendship between the US and Iran.