WATCH: Video from a citizen journalist at today's clashes reportedly shows Basij forces attacking a woman in Isfahan. (Another similar video can be seen here.)
By Golnaz Esfandiari
Heavy clashes have been reported outside an Isfahan mosque where a memorial ceremony was due to be held in honor of Grand Ayatollah Hossein Ali Montazeri, a founding architect of the Iranian Revolution and spiritual father of Iran's opposition Green Movement.
Clashes were also reported in Najafabad, the city where Montazeri was born.
Referring to the city where Montazeri was born, Jaras said: "Sporadic clashes started from Tuesday night in Najafabad and still continued. The situation is tense in the city. People are chanting anti-government slogans."
The clashes come two days after a funeral ceremony for the 87-year-old Montazeri, who died on December 19 in his home in Qom, turned into a huge antigovernment protest in the holy city located southwest of the capital.
Eyewitnesses tell RFE/RL that as mourners and opposition supporters arrived for today's memorial service, they found security forces and plainclothes agents awaiting them in front of the Isfahan mosque.
According to some reports, including on reformist websites, police used force and tear gas to disperse the crowd of Montazeri supporters.
"There was a huge crowd," said one witness who spoke to RFE/RL's Radio Farda on condition of anonymity. "There were clashes between the people and those who were ready to prevent [the memorial] from taking place."
Opposition websites were reporting that many mourners were injured, and up to 50 arrested, but those claims could not be verified.
Iranian authorities have severely restricted journalists' access to events amid continuing postelection tension.
Iran's police chief warned today that "illegal" activities will be met with a "fierce" response from authorities, according to Fars news agency.
"We advise this movement to end their activities," Esmail Ahmadi Moqadam was quoted as saying. "Otherwise, those who violate the order will be fiercely confronted, based on the law."
The reformist website Jaras was reporting that security forces had surrounded the Isfahan home of leading reformist cleric Ayatollah Jalaleddin Taheri, who was to lead today's memorial ceremony.
Jaras also reported that "sporadic clashes started from Tuesday night in Najafabad and still continued. The situation is tense in the city. People are chanting antigovernment slogans."
But Ahmad Khaleghi, a close aide to Ayatollah Jalaleddin Taheri, told RFE/RL's Radio Farda that there was a mourning ceremony in Najafabad for Montazeri this afternoon and that everything had gone well.
"The forces of the Revolutionary Guards and plain-clothes agents didn't get involved in the ceremony in Najafabad and because of that everything went well," Khaleghi said. "But unfortunately in Isfahan the special habits of some of the people and their oversensitivities regarding the late grand ayatollah led them to physically attack the people.
"I saw with my own eyes that they arrested at least 20 people in Isfahan. One example is Majid Esleki, who is the brother of two martyrs. We have reports that he was seriously injured and that he's currently held at the police hospital."
Powerful Symbol
Montazeri was an architect of Iran's 1979 Islamic revolution who went on to became one of the fiercest critics of the clerical establishment. He was once seen as potential successor of Ayatollah Ruhollah Khomeini as supreme leader. But after Ali Khamenei was chosen to take the post following Khomeini's death in 1989, Montazeri's outspoken views led to persecution.
In the late 1990s, Montazeri was placed under house arrest and, although it was lifted in recent years, he was still kept in check by the authorities.
Iranians hold portraits of Grand Ayatollah Hossein Ali Montazeri during his funeral in Qom on December 21.
Most recently, Montazeri condemned the brutal postelection crackdown that followed Iran's June presidential election, and accused the government of dictatorship. 
His funeral this week turned into one of the largest protests seen by opposition forces since the disputed voting. Opposition supporters and mourners chanted slogans against Supreme Leader Ayatollah Ali Khamenei as well as President Mahmud Ahmadinejad.
Witnesses in Isfahan said today that mourners chanted slogans against Khamenei, including "Khamenei is a murderer; his rule is null."
The supreme leader is Iran's ultimate arbiter in religious and political matters under the country's postrevolutionary constitution.
"What happened today in Isfahan is unprecedented regarding the turnout of the people and also the slogans that people chanted," said the eyewitness who requested anonymity. "We haven't had anything similar to this in previous gatherings."
Opposition Momentum?
The death of Montazeri, who was widely respected among Iranians, appears to have re-energized the opposition movement, which has staged a number of protests in various cities throughout Iran since his death was announced.
Morteza Semyari, one of the leaders of Iran's largest reformist student group, Daftar Tahkim Vahdat (Office to Foster Unity), told RFE/RL that Montazeri's legacy will remain alive within the opposition movement.
"What happened in Qom [at Montazeri's December 21 funeral service] demonstrated that the ideas of Montazeri will remain alive," Semyari said. "The movement that we saw in Qom is intertwined with the principle and views of Montazeri, and it will move forward."
On December 22, hard-liners, who generally act with at least implicit support of the government, attacked the home of one of Montazeri's sons, breaking windows.
They also attacked the home of a senior pro-reform cleric, Ayatollah Yusuf Sanei, whom some describe as Montazeri's possible replacement.
Montazeri's eldest son, Ahmad Montazeri, told RFE/RL he believes hard-liners were threatened by the throngs of mourners who came out his father's funeral.
"Millions of people attended the December 21 funeral and it was magnificent, despite the poisonous campaign of the past 20 years against Ayatollah Montazeri," Ahmad Montazeri said. "When they [hard-liners] saw such support from the people -- 90 percent of them youth that came to Qom from places far and near -- they were shocked and they reacted [angrily]."
More protests are expected on December 27, which will mark the seventh day since Montazeri's death and will coincide with the major Shi'ite religious holiday of Ashura.
Authorities launched a brutal clampdown after the June election dispute that has included killings, mass arrests, and the alleged rape and torture of detainees.
But Green Movement leader Mir Hossein Musavi, the presidential runner-up whom supporters fear could be targeted with arrest, has pledged that reform-minded Iranians cannot be silenced by force.
Radio Farda broadcasters Mossadegh Katouzian and Roya Karimi contributed to this report