President Donald Trump called out the Iranian regime for producing 40 years of "corruption", "repression" and "terror" in a tweet Monday on the anniversary of the country's 1979 Islamic Revolution.
This year's celebrations come amid heightened tensions with the United States, which last year withdrew from the 2015 nuclear agreement between Tehran and world powers and reimposed tough sanctions on the Iranian economy.
Crowds were marching on Monday, heading out from a dozen points in the Iranian capital toward Tehran Azadi, or Freedom Square.
The large turnout at these state-sponsored rallies comes as Iranians faces rising prices and food shortages which have triggered waves of protests.
The crowd of demonstrators in the rallies paid tribute to the late founder of the Islamic Republic, Imam Khomeini, under whose leadership the revolutionary movements triumphed over the despotic regime of Pahlavi on February 11, 1979.
There's also a huge display of domestically-developed military equipment, including drones, missiles, bombs, and weapons.
President Hassan Rohani blasted a USA "conspiracy" against Iran as the country marked the 40th anniversary of the 1979 Islamic revolution that ended the monarchy and heralded the start of four decades of clerical rule.
"US should take note: REAL Iranians never succumb to diktats".
Rouhani was referring to the Kingdom of Bahrain, which Iranian officials in the past have referred to as the "14th province". But he remained defiant as Iranians recalled the end of a monarch who catered to the rich and unleashed secret police on dissenters.
"The Iranian people have and will have some economic difficulties [due to the sanctions] but we will overcome the problems by helping each other", Rouhani said in his address.
The 10-day period from the return of Imam Khomeini until the revolution's victory is celebrated annually in Iran, and is known as the Ten-Day Fajr (Dawn).
Israeli Prime Minister Benjamin Netanyahu dismissed the threat. "But if they attack us, we will raze Tel Aviv and Haifa to the ground", Javani told the state news agency IRNA.
In a report coinciding with the celebrations, Reporters Without Borders (RSF) said that 61,900 political prisoners had been held in Iran since the 1980s.
Washington and its Arab allies have viewed Iran with great suspicion since the Islamic Revolution, fearing Khomeini's radical ideology would inspire militants across the Middle East.
A senior commander in Iran's Revolutionary Guards said Tehran would not withdraw its forces from the region, dismissing USA calls that Iranian regional influence should be curbed. -Gen. Hossein Salami, deputy head of Iran's Revolutionary Guards.