On February 11, 1979, Iran's army declared its neutrality in the dispute between the two sides.
On Monday, Iranian state television broadcast images of people carrying Iranian flags and standing out in cold, rainy weather.
The US officially removed MEK, which describes itself as the biggest rival to the ruling Islamic Republic authorities, from its terrorism blacklist in 2012.
"However, this would be the last anniversary of the revolution that they celebrate".
"The 40th anniversary of the Iranian regime only serves to highlight four decades of failure and broken promises".
"40 years of corruption".
One sign read, "Much to the dismay of America, the revolution has reached its 40th year". 40 years of terror.
Journalist Mikey Kay recalled in this vein that Bolton has spoken at least eight times at rallies organised by MEK (Mojahedin-e Khalq), an Iranian dissident group which calls for regime change in Iran and is known to pay up to $50,000 per speech.
This year's anniversary come as tensions are rising with the USA and Iran is grappling with re-imposed US sanctions. "However, this would be the last anniversary of the revolution that they celebrate".
Reuters could not independently verify the pictures.
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Iranians are facing mounting economic hardships which many blame on the country's clerical leaders.
This year's anniversary comes as Iran deals with the aftermath of President Trump's decision in May to withdraw from the 2015 nuclear deal and restore tough US sanctions.
Iranian state TV anchor Mehdi Khosravi chirped how he expects John Bolton, U.S. national security adviser, to be very angry on Monday since he had once predicted that Iranians will not see the 40th anniversary of their revolution.
In January, President Hassan Rouhani said Iran was facing its worst economic crisis since the Shah was toppled.
He said, "We will not let America become victorious.Iranian people have and will have some economic difficulties but we will overcome the problems by helping each other".
People from different walks of life have attended the processions with flags of Iran and banners in support of the Islamic Republic to mark the revolution's ruby jubilee.
"The United States does not have the courage to fire a single bullet at us despite all its defensive and military assets".
Israeli Prime Minister Benjamin Netanyahu dismissed the threat. "They should take this into account", Netanyahu said in a warning to his country's archfoe.
The Shah was the United States' closest ally in the Middle East.
Cleric Hossein Firouzi told AFP Iran's revolution had achieved everything it set out to in terms of military power, political identity and scientific achievements.