You may think the attention in Iran this weekend will be focused squarely on the backroom happenings in Vienna, where Iranian and Western diplomats are wrangling over the Islamic Republic’s controversial nuclear program. But you would be wrong.

On Sunday, millions of Iranians — perhaps even as much as one-quarter of the country’s population — will tune in to watch the soccer match between Persepolis and Esteghlal, the country’s two biggest clubs. Both are based in Tehran, and the city’s main Azadi Stadium will be packed to its 80,000 capacity, divided between the red of Persepolis and the blue of Esteghlal.

Soccer is the most popular sport in the country and the fervor that surrounds these clubs — as well as the rivalry between them — is legendary.

… That sort of chaos is an obvious concern for the mullahs, who would never let dissidents convene on the same scale as sports fans. “Soccer poses an opportunity and a threat to a non-democratic regime,” says James Dorsey, an expert on Middle East soccer politics, in an interview with Bloomberg. “Soccer can easily become a platform for protest because the emotions it evokes are tribal.”