Iranian security forces have clashed with opposition supporters in central Tehran, according to Jaras, an Iranian opposition website.
The confrontation comes as the Islamic Republic marks Ashoura, a 10-day period of Shia religious ceremonies.
The report could not be independently confirmed, as foreign media have been banned from reporting directly on protests.
The same website earlier reported that hundreds of riot police were deployed in central Tehran.
"A large group of people and security forces clashed in the Pol-e Choubi area while people were marching," Jaras said.
The reported clashes occurred as people around Iran marked the day of Tasoua with religious ceremonies which peak on Sunday, when Shia Muslims commemorate the 7th century death of Hussein, Prophet Muhammad's grandson.
Sadegh Zibakalam, a professor at Tehran University, said the opposition had defied security forces' warnings against using Tasoua as an occasion to stage anti-government demonstrations.
"It appears that the opposition is not something that we can say has disappeared or been reduced. The opposition has used every occasion to register its voice," he told Al Jazeera.
Six months after a disputed election plunged Iran into political turmoil, tension has again mounted after the death of Grand Ayatollah Hossein Ali Montazeri, a leading dissident religious leader, last week.
Mourning and burial ceremonies for Montazeri, who died aged 87, have sparked bloody confrontations with security forces.
Iranian police clashed on Thursday with Montazeri mourners, making arrests and injuring some, an opposition website said after Tehran warned of a crackdown.
Rehesabz.net said the incident happened in the northern city of Zanjan after authorities reportedly banned most memorial services for Montazeri after his funeral turned into an opposition rally.
"People gathered for Ayatollah Montazeri's memorial were faced with locked mosque doors and decided to hold the ceremony in the street," Rahesabz.net said.
"People were mourning in silence and the Quran was being recited but the police sought to disperse them which led to clashes and people were severely beaten as they were running away.
"Many have been arrested and some of the injured have been taken to hospital."
The report could not be independently verified as foreign media are banned from covering opposition-linked gatherings.
'Person of the year'
In another development that could possibly anger Tehran, a British newspaper on Saturday named an Iranian woman shot dead during protests against her country's disputed June elections as its "person of the year".
The Times said Neda Agha-Soltan became a "global symbol of opposition to tyranny" after images of her bleeding to death during the protests in Tehran were shown around the world.
"Ms Soltan, 26, joined the protest because she was outraged at the way that the regime stole the presidential election," the newspaper said on its front page that included a photograph of protesters holding pictures of her.
"She wanted to make a difference, she said. She had no idea quite what an impact she would have. Mobile phone footage of her bleeding to death on a pavement flashed around the world.
"It tore the last shreds of legitimacy from the regime, made her a global symbol of opposition to tyranny, and inspired the Green movement in a region where populations are all too easily cowed."