Waving flags of Iran and holding portraits of the founder of the Islamic republic, the late Ayatollah Ruhollah Khomeini, they converged on Tehran's Azadi Square despite chilly temperatures.
'Death to America'
State TV showed video footage of rallies in at least half a dozen cities outside the capital, including Mashhad, Ahvaz and Kerman, with people holding signs that read, “Death to America” and “Death to Israel”.
Khomeini's return from exile
The celebrations mark the day that Shiite cleric Khomeini returned from exile and ousted the Shah's last government.
The state has appealed for a strong turnout as a show of solidarity after a year in which Iran has been shaken by protests and military tensions with the United States.
'Symbol of unity'
"Securing our country and our region depends on our unity, and participation in this rally is a symbol of this unity," Hadi Khamenei, brother of Iran's supreme leader Ayatollah Ali Khamenei, said on state television.
Celebration amid US conflict
Iran almost got into a full-blown conflict with the United States last month after a US drone strike killed top Iranian military commander Qassem Soleimani in Baghdad on January 3, prompting Iran to retaliate with a missile barrage against a US base in Iraq days later.
Tensions continue to spike
Tensions spiked between Iran and the United States after US President Donald Trump withdrew from a multilateral nuclear deal with Iran in 2018 and reimposed sanctions in a bid to pressure Tehran to negotiate over its ballistic missile programme and ties with regional proxy groups.