A U.S. State Department official says "Iran is increasingly showing itself to be a police state" in its harsh treatment of opposition protesters.
State Department Spokesman P.J. Crowley told reporters Wednesday that Iran is using its security forces to try to "stamp out" the "aspirations of the Iranian people."
On Wednesday, Iranian opposition groups say security forces armed with tear gas and batons clashed with supporters of the late dissident cleric, Grand Ayatollah Hossein Ali Montazeri, as they rallied in central Iran.
Iranian police official Esmail Ahmedi-Moghadam told journalists the opposition must "stop its illegal activities" or police would stop them.
When asked about the clashes, Iranian President Mahmoud Ahmadinejad told Britain's Channel 4 News that "the law prevails" and must be observed and respected during protests in Iran.
In the interview broadcast Wednesday, President Ahmadinejad also said the people of Iran are united and determined to protect their rights, interests and independence.
Opposition Web sites say Wednesday's clashes took place at gatherings in memory of Ayatollah Montazeri in Isfahan and his birthplace of Najafabad.
The opposition reports say security forces in Isfahan arrested more than 50 demonstrators and injured many others. Amateur video appears to confirm the accounts of violence.
Iran has banned foreign media from covering protests directly.
Ayatollah Montazeri died Sunday in Qom at the age of 87.
He was considered a spiritual patron of Iran's reformist movement and a strong critic of the nation's conservative clerical establishment.
Opposition groups said hundreds of thousands of mourners joined a funeral procession for the ayatollah in Qom Monday that evolved into a major anti-government protest.
Government supporters in Qom staged a smaller counter-demonstration on Tuesday.
Some information for this report was provided by AP, AFP and Reuters.
