The last time Iran had street demonstrations this big was 30 years ago. In 1979, Iranians deposed U.S.-backed dictator Mohammad Reza Shah Pahlavi and established the Islamic Republic.
"These are the most significant demonstrations in the history of the Islamic Republic over the past 30 years," said Afshin Molavi, a former Reuters journalist who is now a scholar at the New America Foundation, a Washington, D.C.-based nonprofit, nonpartisan public policy institute.
Molavi called the demonstrations "eerily reminiscent, in terms of numbers, of the demonstrations that led to the overthrow of the Shah of Iran."
The revolution gave Iran an elected president. But real power actually rests in the hands of Iran's Shiite Muslim clerics, led by Ayatollah Ali Khamenei. The handpicked successor to the ayatollah who led the revolution, Khamenei has sweeping powers.
He has absolute authority over the institutions of government, including the judiciary and the armed forces. He appoints military commanders, directors of state media organizations and the country's religious leadership.
Khamenei also has power over presidential elections, the source of the current dispute after incumbent President Mahmoud Ahmadinejad was declared the winner less than an hour after the polls closed, even though the 40 million votes were to be counted by hand.
Officially, Khamenei does not favor one presidential candidate over the other, but he and the main opposition leader, Mir Hussein Moussavi, have clashed in the past.
The struggle, playing out with protests in the streets, reflects the power struggle between Moussavi and Khamenei.
Moussavi is no Nelson Mandela -- he's a former prime minister and a member of the Islamic old guard -- but he has surprised many Iran watchers with his willingness to challenge the ayatollah.
"When you think of Moussavi, think [former Soviet leader Mikhail] Gorbachev," Molavi said. "He is not a revolutionary who wants to firebomb the Islamic Republic. He is someone who wants to work within the system to change the system."
"Moussavi, were he to become president of Iran, would not simply bow and acquiesce to the ayatollah. There would be some tension in that relationship," Molavi said.
The protests alone are a sign of that tension. But it's not clear how far Moussavi is prepared to press his case. If the authorities crack down even harder, it's not clear whether that will intimidate the protesters, or inflame them.
"The supreme leader is facing an incredibly delicate situation," said Susan Maloney, who studies Iran for the Brookings Institution. "He is facing not only a rebellion on the streets of all of Iran's cities, but also intense pressure behind the scenes from members of the power structure.
Already, Khamenei has made some concessions. He agreed to the recount and is, for the most part, tolerating the protests. Iranian state TV announced Tuesday that he also met with envoys of four opposition candidates and called for unity.
Whether Ahmadinejad or Moussavi wins the presidency, Khamenei will still be the supreme leader and will be the most powerful person in Iran.
Iran-watchers say this may well be a tipping point, but it also has the potential to become Iran's Tiananmen Square, a showdown that would be a setback for political reform.