FIFA banned the pre-revolutionary Lion and Sun flag, which has become a widely used symbol among opponents of the present government.
But protesters still managed to sneak the flags into the Los Angeles Stadium, while others wore shirts bearing its colours.
Footage shared online appeared to show stadium security confiscating some flags.
In other videos, protesters were seen carrying separate pieces of fabric in the colours of the historic Iranian flag, apparently to avoid detection, before stitching them together inside the stadium.
Fans hold the old flag of Iran aloft during the match with New Zealand despite FIFA banning it. (Getty Images)Getty Images
When the Islamic Republic’s anthem was played, hundreds of Iranians turned their backs.
Chants of “Death to the Islamic Republic” and “Long live the king” could also be heard, the latter a slogan used by supporters of Crown Prince Reza Pahlavi, the son of Iran’s late Shah and a leading opposition figure in exile.
Iranian opponents of the regime were also heard cheering on New Zealand alongside the Kiwi fans.
Among those present was Mersedeh Shahinkar, a member of the Woman, Life, Freedom protest movement, who lost an eye after being shot by Iranian security forces.
On Instagram, she posted several videos of herself and other protesters celebrating New Zealand’s attacks against Iran on the pitch.
Supporters of the Islamic Republic were also present inside the stadium, some wrapped in the regime’s official flag.
Videos appeared to show confrontations between them and anti-regime demonstrators.
In one clip, a woman wearing a hijab was shouted down by protesters after allegedly expressing support for the regime’s crackdown on demonstrators earlier this year.
In the match, New Zealand were 2-1 up until just after the hour mark when Iran equalised.
After finding the net, Mohammad Mohebi raised his hand in the shape of a gun and pointed it towards the crowd.
Images of the gesture quickly spread on social media, where Iranian anti-regime activists interpreted it as a threatening message aimed at opponents of the Islamic Republic, many of whom had been barred from openly carrying Iran’s historic flag in an effort, they said, to prevent the match from becoming politicised.
Iran drew 2-2 with New Zealand (Getty Images)Getty Images
Iran ultimately failed to beat what was widely regarded as the weakest team in Group G, drawing 2-2 with New Zealand in a result that denied them the chance to move top of the group after the opening round of matches.
