“The Iranian nuclear issue has become the main pillar of the regime’s legitimacy,” said Mustafa el-Labbad, an Iran expert in Cairo. “So Ahmadinejad is putting it in the center of the scene in order to conceal the internal differences and huge domestic challenges they face.”

It is not at all clear how much uranium has been enriched or whether Iran has the capacity to transform that into either weapons-grade fuel or even into the fuel rods necessary for running the medical research reactor it says it aims to supply.

The Obama administration dismissed the idea that Iran had the capacity to enrich uranium to weapons grade. The White House press secretary, Robert Gibbs, said, “Iran has made a series of statements” that were “based on politics, not on physics,” The Associated Press reported.

Mr. Ahmadinejad’s speech, delivered from a platform decorated with flowers, was heard by a largely sympathetic crowd that had been bused in from around the country to celebrate the most important political celebration of the year. The day is similar to the Fourth of July in the United States or Bastille Day in France, and like those holidays it is steeped in myth and symbolism.

Both the opposition and the government hoped to control the message of the day. The opposition leaders, Mir Hussein Moussavi and Mehdi Karroubi, had called on their supporters to fill the streets, though they had not offered a clear plan.

The government provided little room for the opposition to gain traction. It sent thousands of security officers into Azadi Square the night before to intimidate the opposition and, if necessary, block it from participating. Those who attended the rally disappeared in a sea of government supporters.