In the space of 24 hours, Tehran's Valiasr Square sees baton-wielding cops and a presidential victory celebration.

TEHRAN, Iran—Events here over the last two days have highlighted the vast differences in how supporters of incumbent President Mahmoud Ahmadinejad and those of his main rival, Mir Hossein Mousavi, view the official results of Friday's election.

From a logical point of view, the results seem dubious at best. Ahmadinejad is reported to have received well over 60 percent of the vote, compared with just more than 30 percent for Mousavi. Given the large number of Azeri Turks (Mousavi's ethnicity) in Iran, the number of people in major cities campaigning for him—and, perhaps most important, the number of women mobilized by his outspoken wife, Zahra Rahnavard—the race seemed a lot tighter than the final tally would indicate.

A feeling of dejection hung in the air for most of Saturday. Spontaneous street demonstrations early in the day were small and were quickly broken up by riot police on motorcycles.

As reality set in, people began taking to the streets en masse. Around 5 p.m. on the approach to Fatemi Square, where the Interior Ministry is located, I could see that the entire traffic circle had been closed to car traffic. About 200 riot police waited in the middle of the square. I headed down an alley, just steps away, where protesters had created a blockade of flaming garbage cans.

The demonstrators pushed aside a garbage can, opening a path, and rushed forward. Simultaneously, baton-wielding police charged. The protesters hurled rocks, and the police responded by beating everyone who couldn't escape into one of the connecting alleys.

Citizens, nearly all on the side of the protesters, left their front gates open just a little to offer those of us fleeing the police an escape route.

As we caught our breath in someone's driveway, I asked a man in his mid-30s whether he had witnessed anything like this before. "Over the last two weeks," he told me, "between the debates, the number of people in the street last week, and the violence now—no. I've never seen anything like this."

Along with the anger, there was still a sense of excitement and exuberance, as though the nation were releasing frustrations bottled up for decades.

Just to the south, above Valiasr Square, one of Tehran's major commercial hubs, lines of protesters chanting "Death to the dictator" blocked traffic on the city's busiest street. In every direction, small groups of four or five congregated to discuss what they'd seen, sometimes dispersing when the police began to move in.

A woman who was trying to cross the avenue was shoved onto the sidewalk by a member of the Basij militia, who spat at her, "We will kill those of you who come into the street!" As she walked away, she exclaimed in disbelief, "They steal our vote and then they talk to us like that?"

Chaos erupted again as people fled in all directions. As they attacked the crowds with their batons, the Basij responded to the protesters' shouts of "Death to the dictator!" with cries of "God is great!"

I approached an elderly man who seemed riveted but disgusted by the scene unfolding in front of him. "I've never seen violence like this before," he said, "What can you expect when you disrespect the people? This is a coup d'état. After blatantly cheating the people, they won't be able to turn this off."

Similar riots took place throughout Tehran Saturday night.