Hard-liners attacked senior pro-reform leaders in the streets as tens of thousands marched in competing mass demonstrations by the opposition and government supporters. Opposition protesters, chanting "death to the dictator," hurled stones and bricks in clashes with security forces.

The opposition held its first major street protests since mid-July, bringing out thousands in demonstrations in several parts of the capital. In some cases only several blocks away, tens of thousands marched in government-sponsored rallies marking an annual anti-Israel commemoration.

The commemoration, known as Quds Day, is a major political occasion for the government — a day for it to show its anti-Israeli credentials and its support for the Palestinians. During a speech for the rallies, President Mahmoud Ahmadinejad railed against Israel and the West, questioning whether the Holocaust occurred and calling it a pretext for occupying Arab land. Quds is the Arabic word for Jerusalem.

But the opposition was determined to turn the day into a show of its survival and continued strength despite a fierce three-month-old crackdown against it since the disputed June 12 presidential election.

The four top opposition leaders joined the protests, in direct defiance of commands by Supreme Leader Ayatollah Ali Khamenei, who barred anti-government demonstrations on Quds Day. That could provoke an escalation in the crackdown: hard-line clerics have been demanding the past week that any leader backing the protests should be arrested.

Tens of thousands joined the government-organized marches, starting in various parts of the capital and proceeding to Tehran University. Police and security forces, along with pro-government Basij militiamen, fanned out along main squares and avenues and in many cases tried to keep nearby opposition protesters away from the Quds Day rallies to prevent clashes, witnesses said.

But at one of the several opposition rallies around the city, a group of hard-liners pushed through the crowd and attacked former President Mohamad Khatami, a cleric who is one of the most prominent pro-reform figures, according to a reformist Web site. The report cited witnesses as saying the opposition activists rescued Khatami and quickly repelled the assailants.Another reformist Webs site said Khatami's turban was disheveled and he was forced to leave the march.

Hard-liners tried to attack the main opposition leader, Mir Hossein Mousavi, when he joined another protest elsewhere in the city, a witness said. Supporters rushed Mousavi into his car when the hard-liners approached, and the vehicle sped away as his supporters pushed the hard-liners back, the witness said. He and other witnesses spoke on condition of anonymity for fear of government retaliation.

In one of the main Tehran squares, Haft-e Tir, baton-toting security forces tried to break up one of the opposition marched, and were met with protesters throwing stones and bricks, witnesses said. Several policemen were seen being taken away with light injuries. At least 10 protesters were seized by plainclothes security agents in marches around the city, witnesses said.

The opposition claims that Ahmadinejad won the June election by fraud and that Mousavi is the rightful victor. Hundreds of thousands marched in support of Mousavi in the weeks after the vote, until police, Basij and the elite Revolutionary Guard crushed the protests, arresting hundreds. The opposition says 72 people were killed in the crackdown, thought the government puts the number at 36. The last significant protest was on July 17.

On Friday, opposition supporters poured out on the streets in green T-shirts and wearing green wristbands — the color of the reform movement — and marched with fingers raised in the V-sign for victory, chanting "Death to the Dictator."

Others shouted for the government to resign, carried small photos of Mousavi, while some women marched with their children in tow.

There were also chants of: "Not Gaza, not Lebanon — our life is for Iran" — a slogan defying the regime's support for Palestinian militants in Gaza and Lebanon's Hezbollah guerrilla.

Two other opposition leaders appeared at the protests — Mahdi Karroubi, who also ran in the June election, and former President Akbar Hashemi Rafsanjani, according to the semiofficial Fars news agency. Rafsanjani is a senior cleric in Iran's leadership but has been a behind-the-scenes supporter of Mousavi.

His appearance at the rally is a rare overt show of backing for street protesters. It comes after Rafsanjani was banned this year from his customary role delivering the Friday prayers on Quds Day, which he has done the past 25 years. On Friday, the prayer sermon was delivered by a hard-line supporter of Ahmadinejad, Ahmad Khatami.

In sheer numbers, the opposition turnout was far smaller than the mass pro-government Qods Day marches — not surprising given the state's freedom to organize the gathering. Customarily on Quds Day, Tehran residents gather for pro-Palestinian rallies in various parts of the city, marching through the streets and later converging for the prayers ceremony. The ceremony was established in 1979 by the leader of the Islamic Revolution and founder of present-day Iran, Ayatollah Ruhollah Khomeini.

Just hundreds of yards (meters) away from opposition protesters on the main Keshavarz Boulevard, thousands of Ahmadinejad supporters marched carrying huge photographs of the president and Supreme Leader Khamenei. Some in the government-sponsored rally chanted: "Death to those who oppose the supreme leader!"

At the climax of the occasion, Ahmadinejad addressed worshippers before Friday prayers at the Tehran University campus, reiterating his anti-Holocaust rhetoric that has drawn international condemnation since 2005. He questioned whether the "Holocaust was a real event" and saying Israel was created on "false and mythical claims."