JIM LEHRER: Thousands of Iranians defied a government ban today to protest what they called a rigged presidential election. They gathered in central Tehran to support the pro-reform candidate.
The protests were largely peaceful, until later in the day when pro-government militia fired at opposition demonstrators, killing at least one. There were also reports of gunfire in three other parts of the capital city.
Bill Neely of Independent Television News begins our lead story coverage.
BILL NEELY: He hasn't been seen since the election he was widely tipped to win. Today, Mir Hossein Mousavi appeared before hundreds of thousands of his supporters.
They are convinced he was robbed. They want a rerun, and they swamped Tehran today in the biggest protest march here in 30 years.
Mr. Mousavi told them he was ready for a new election and will not surrender until he gets one. The protesters carried signs comparing President Ahmadinejad with a dictator. And everywhere the question: Where is our vote?
IRANIAN PROTESTER: It's not election. It's lie election. It was a fraud election.
BILL NEELY: These protesters are taking a defiant, but very dangerous stand. This demonstration has been branded not only illegal or criminal, but treacherous against the system, and the police have been authorized to use live fire. This is a high-stakes showdown.
Thousands of riot police were out, batons at the ready. They've broken up just about every protest demonstration so far. This one may be no different.
Are you worried that they will shoot?
IRANIAN PROTESTER: Yes, I'm sure they will do that.
BILL NEELY: You're sure?
IRANIAN PROTESTER: Yes.
BILL NEELY: As they marched, Iran's supreme leader, Ayatollah Khamenei, announced an investigation into the election. The results will be known in 10 days. Demonstrators say they'll keep marching.
Mr. Ahmadinejad, who said the protesters are like disappointed football fans whose team has lost, would not ensure his rival's safety when I challenged him.
Sir, can you guarantee Mr. Mousavi's safety?
His silence places his rival in some danger. Mr. Mousavi's wife has now publicly called President Ahmadinejad a dictator, dangerous words.
A second defeated candidate I spoke to says the election results are so ridiculous and unbelievable it can't be expressed in a single sentence.
Officially, Mr. Ahmadinejad won 2 out of every 3 votes cast, but no detailed breakdown has been published. He apparently won by a landslide even in his rival's home city and home province.
IRANIAN PROTESTER: It's a big lie.
BILL NEELY: It's a big lie?
IRANIAN PROTESTER: I can't accept it. I can't.
BILL NEELY: It's now a trial of strength, protesters shouting, "Death to the dictator," against well-armed riot police with clear orders now to crush dissent. Hundreds of people have been beaten or arrested, including leading reformers and politicians.
The vicious running battles by day and night have fueled an explosive situation, pent-up desire for change bursting in the capital. The stakes are high because this is a direct challenge not just to a disputed election and a president, but to the whole regime.