It is the 38th anniversary of the Islamic revolution in Iran, and it comes at a moment when relations with the USA have plunged.
The election of Donald Trump in America, who has warned Iran it is “on notice” and who says the nuclear deal with struck with former president Barack Obama should be torn up, is seen as a step backwards for bilateral relations in Tehran.
“In the United States, there are some who are newly in power. Who threatens our government and our armed forces has to know that our people is united, vigilant and will resist the enemy until the end,” said President Hassan Rouhani.
The atmosphere among the hundreds of thousands of people who are gathering in Azadi square is defiant. Few people here think the deal will be scrapped, and the mood was confident and cheerful.
The banners on display criticised the usual suspects of Britain, America and Israel, but among them were also messages of thanks to the American people for opposing Trump’s executive order banning travellers from seven mainly Muslim countries, among them Iran.
A hashtag, #LoveBeyondFlags, is also prominent in social media today, where some Iranians are urging demonstrators to stop burning flags during the anniversary.