Waving the flag used by Iran before the 1979 Islamic revolution, demonstrators beat drums and chanted slogans denouncing the national side, which they insist is a propaganda tool for Tehran.
“This team is not the Iranian people’s team, it’s the regime’s team,” said Ava Amin, a philosophy student who came to demonstrate with a banner calling for “regime change.”
“When the people get murdered, they ignore it and stay silent,” she told AFP.
The match, which ended in a 2-2 draw, took place under tight security, with a large contingent from the Iranian-American community in Los Angeles vowing to make known their opposition to the regime.
The city - sometime dubbed “Tehrangeles” - is home to the largest ethnic Iranian community outside of Iran, with a sizeable number made up of those who fled around the time of the revolution, or their descendants.
Some of those people say they want to use the visibility of the World Cup to highlight the abuses committed by the military-backed clerics who have held power in Iran for 47 years.
Some protesters held up collages of portraits of Iranians who died during the bloody crackdown on anti-regime protests in January - a crackdown that claimed thousands of lives, according to numerous NGOs.
“We lost so many people in January,” Amin said. “When people ask for freedom in Iran, they get killed, so we have to be here. We’re here to be their voice and raise our flag.”
Three hours before the match, AFP spoke with several protesters who held tickets and wore T-shirts printed with that pre-revolution flag - horizontal green, white and red stripes overlaid with a sun and a lion.
Many said they planned to conceal the symbol to smuggle it into the stadium.
FIFA regulations prohibit the display of political symbols at venues and Tehran officials have threatened to halt the match if the flag is seen.
Some Iranian-Americans inside the stadium told AFP that protesters should keep politics out of football.
“It is sport! It is not something political. They can go through the street and talk and do whatever they want. But here is not the place,” Farideh Mansoor said.
The players “did everything to get here. That’s why we have to support them. We respect them, we support them, and we wish the best for them.”
The controversy around the squad comes on top of the many tensions surrounding the build-up to the tournament, which is being jointly hosted by the United States, Canada and Mexico.
At the end of February, the US and Israel launched attacks on Iran, which hit back at US allies in the Gulf and choked off the Strait of Hormuz, strangling oil supplies and denting the global economy.
On Sunday, the US and Iran announced an outline deal to end the war.
The Iranian team’s participation had been clouded by the hostilities, and an original plan to base their training camp in Arizona was nixed in favor of a location in Tijuana, just over the Mexican border.
All three of Iran’s group stage matches take place in the United States.