Thousands of Iranians massed on Tuesday for commemorations marking 41 years since the Islamic Revolution, in a show of unity at a time of heightened tensions with the United States.

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" and "We will resist until the end," read some of the banners carried by those in the crowd.

"Iran is looking forward to creating another epic," read the news ticker on state television, which called for a massive turnout.

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.

"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.

A tank outside Niavaran palace in 1979 with a picture of the late Ayatollah Ruhollah Khomeini on the front during the Islamic Revolution which saw the ouster of the shah's last government
A tank outside Niavaran palace in 1979 with a picture of the late Ayatollah Ruhollah Khomeini on the front during the Islamic Revolution which saw the ouster of the shah's last government AFP / GABRIEL DUVAL

Iran's economy has been battered since US President Donald Trump in 2018 abandoned an international nuclear deal and reimposed sanctions and a stated campaign of "maximum pressure".

When Iran raised petrol prices in November, nationwide protests erupted and turned violent before security forces put them down amid a near-total internet blackout.

Tensions with Washington escalated in early January when a US drone strike killed powerful Iranian general Qasem Soleimani in Baghdad.

Iran retaliated by targeting US forces but then accidentally shot down a Ukrainian airliner, killing all 176 people on board, in a tragedy that sparked anger at home and abroad.

Soleimani's daughter Zeinab was to address the crowd at Azadi Square on Tuesday, followed by President Hassan Rouhani.

This year's anniversary also comes ahead of crucial parliamentary elections in Iran.

The alliance of moderates and reformers that propelled Rouhani to power in 2013 is scrambling to avoid losing its majority in the February 21 election.

Rouhani's government has come under intense pressure from conservatives for agreeing the 2015 nuclear deal that has unravelled since Trump's decision to withdraw from it and reimpose crippling sanctions.

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