WATCH: YouTube video appearing to show street fighting between Iranian riot police and protesters in Tehran on December 27.
By Golnaz Esfandiari
At least eight people have been killed in some of the bloodiest confrontations in months between Iranian opposition supporters and security forces.
The state Press TV said eight people died in violence in Tehran and police said more than 300 people were arrested.
Those reported killed in the capital included a nephew of former opposition presidential candidate Mir Hossein Musavi.
Reports from today say police fired teargas to disperse Musavi supporters who had gathered to express their condolences over the death of his nephew.
"A group of Musavi supporters have gathered in front of Ebn-e Sina hospital where his nephew's body was kept...Police fired teargas to disperse them," the Norooz website reported.
The latest violence is the most deadly since the unrest that immediately followed June's disputed presidential election, in which dozens of people were killed.
The events coincided with commemorations of the Shi'ite Ashura festival and the seventh day of mourning for leading dissident cleric Grand Ayatollah Hossein Ali Montazeri, who died one week ago at the age of 87.
In a statement on an opposition website, opposition leader Mehdi Karrubi asked how authorities could spill the blood of innocent people during Ashura.
The reports of deaths, as well as a YouTube video purporting to show demostrators carrying a gunshot victim, could not be immediately verified.
But graphic videos and photographs of serious injuries have emerged, appearing to confirm some of the worst fears.
The "Parlimannews" website -- which has ties to reformists in Iran's parliament -- reported that Seyed Ali Musavi, the opposition leader's nephew, had been killed during a confrontation with security forces. The report said the younger Musavi had been "shot in the heart" during "Ashura Day events" and died en route to a hospital.
A Musavi aide who requested anonymity after being contacted by RFE/RL's Radio Farda confirmed the report of the relative's death.
In an open letter posted later addressed to Musavi and posted to his "Kaleme" website, adviser Alireza Beheshti expressed his family's condolences over "the martyrdom of your nephew Ali Habibi Musavi."
Government critics have vowed to continue the protests.
A pro-opposition website, "Jaras," said the opposition was organizing more protests in major public parks and in Tehran's Enghelab, Mohseni, Tajrish, and Vanak squares.
Tear Gas And Reported Gunfire
Iranian authorities have banned foreign journalists from many events and imposed tight strictures on domestic media.
Around midday on December 27, an eyewitness told Radio Farda by telephone from Tehran that security forces were using tear gas and pepper gas against opposition supporters to try to disperse them from the city center.
She reported "a big crowd of people" at the intersection of Bozorgmehr and Vali Asr streets.
"The [security forces] on motorbikes attacked [the crowd]. I can see about 100 or 150 of them," the witness told Radio Farda. "People have set fire to several garbage cans. They're trying to chant slogans against the leader of the Islamic Republic [Ayatollah Ali Khamenei] and the dictatorship."
Other reports, mostly from websites sympathetic to the opposition, reported occasional sounds of gunfire.
The clashes come seven days after the death of Iran's longtime dissident cleric, Grand Ayatollah Hossein Ali Montazeri. Montazeri was once in line to succeed revolutionary leader Ayatollah Ruhollah Khomeini but had a falling-out that led to jail time and internal exile.
Montazeri is widely considered the spiritual father of Iran's opposition Green Movement, although two unsuccessful presidential candidates, Mir Hossein Musavi and cleric Mehdi Karrubi, have been the political faces of the resistance that began after the disputed June presidential election.
Witnesses told RFE/RL many of the chants targeted Supreme Leader Ayatollah Ali Khamenei.
Energized Reform Movement?
Montazeri's death has appeared to reenergize the opposition movement, which turned his funeral in the holy city of Qom a week ago into a huge antigovernment protest.
WATCH: YouTube video shows opposition supporters taking to the streets in the Iranian capital on December 27 to protest the disputed June election and the authorities' subsequent clampdown:
Since Montazeri's death last week, protests have taken place in several cities including Tehran, Isfahan, Najafabad and Zanjan.
Protests are also reported today in Isfahan, Montazeri's hometown of Najafabad, Shiraz, and Qom.
On Ashura, Iranians usually march in the streets and beat their chests in memory of the death of the grandson of the Prophet Muhammad, Imam Hossein.
This year, however, many of the Ashura ceremonies in Iran have turned into protests against the Iranian establishment.
A speech on December 26 by former reformist President Mohammad Khatami at the home of the founder of the Islamic Republic, Ayatollah Khomeini, was disrupted by hard-liners.
Khatami, who backed Musavi ahead of the disputed June presidential election as is regarded as a leader of the reform movement, was reportedly drawing parallels between Iran's opposition movement and the struggle of the grandson of the Prophet Muhammad, Imam Hussein, whose martyrdom is being commemorated today in one of Shi'a Islam's holiest days.
WATCH: YouTube video of a crowd of presumed hard-liners forcing their way into a mosque complex where Khatami was addressing a crowd of Montazeri mourners on December 26:
The incident led to protests for several hours until riot police were deployed and dispersed the crowd.
Iran has been rocked by a series of street protests since the June 12 presidential vote and the reelection of President Mahmud Ahmadinejad, which the opposition insists was the result of massive fraud.
Radio Farda broadcasters Mohammad Reza Kazemi and Roozbeh Bolhari contributed to this report