BBC Iran media analyst Saeed Barzin comments on video appearing to show opposition supporters during protests in Tehran
Security forces have used batons and tear gas to disperse opposition supporters in the Iranian capital, Tehran, witnesses and state media say.
Unconfirmed reports said the authorities had also opened fire.
Video footage and photos showed what appeared to be large crowds of opposition supporters being chased by security forces in riot gear.
It came as an officially backed demonstration was held to mark 30 years since the seizure of the US embassy.
Thousands were present at the anti-American rally, about 1.5km (1 mile) from where opposition supporters gathered in Haft-e Tir square.
Other groups turned out in other parts of the capital to voice their opposition to the regime.
Many of the opposition demonstrators wore green scarves or bands, which have been used in repeated protests since Iran's disputed presidential elections in June.
Jon Leyne, BBC Tehran correspondent
With all opposition protests banned in Iran, members of the opposition are using official government demonstrations to get their message across.
Last month, they hijacked the annual Quds Day protests, organised by the government in support of the Palestinians. There have been reports of protests at other public gatherings, such football matches.
The demonstrations have been met by an increasingly strong turnout from the police and the pro-government Basij militia. That has prevented the opposition from gathering together in any one major rally, as they managed to do immediately after the election.
The fact that the protests are continuing at all despite intense government pressure shows the depth of anger over the disputed presidential election and against the government of President Mahmoud Ahmadinejad. If the protests continue, the government's next step may be to arrest key opposition leaders.
Opposition supporters say the elections were rigged to ensure the re-election of President Mahmoud Ahmadinejad.
Riot police and pro-government militiamen had packed the streets in the centre of the capital on Wednesday, after reformist leaders had urged their supporters to attend peaceful rallies at the former US embassy.
In November 1979, 52 US diplomats were taken hostage at the embassy and held for 444 days by Islamist students in support of the Iranian revolution.
A well-known activist who was among the opposition supporters, Habibullah Payman, told the BBC's Persian service that he and others had been attacked.
"[The Green movement] came in peace and with the usual slogan of supporting peace and asking for freedom," he said.
"I saw no sign of violence or agitation in the crowd... Yet, they were attacked violently."
Opposition supporters chanted "death to dictators", as small fires burned in the street. Security forces made a number of arrests, reports said.
Iran's Irna news agency said protesters had attacked a bus, and that two policemen had been injured.
It also reported that security forces had used tear gas in some parts of the city to disperse protesters.
Thousands rallied outside the US embassy in support of the regime
Authorities have placed severe restrictions on foreign news organisations, making it difficult to verify reports.
Iran's Revolutionary Guards had warned opposition groups not to stage demonstrations on the anniversary.
Opposition leader and former presidential candidate Mehdi Karroubi briefly joined the protesters in Haft-e Tir square, according to a reformist website, Mowjcamp.
Unconfirmed reports were circulating that police had surrounded the house of another reformist leader, Mir-Hossein Mousavi, to prevent him from joining any demonstration.
Some arrests were made on Firdowsi Square, Mowjcamp reported.
At least 30 protesters have been killed in clashes and thousands arrested since June's election.
Some 200 opposition activists remain behind bars. Three have been sentenced to death.
On Wednesday's anniversary of the seizure of the US embassy, US President Barack Obama released a statement in which he urged Iran to move beyond the "suspicion, mistrust and confrontation" that had prevailed between Iran and the US since then.
"Iran must choose," the statement said. "We have heard for 30 years what the Iranian government is against; the question now is what kind of future it is for."
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