Tue Jul 21, 1:56 pm ET

TEHRAN, Iran – Security forces and pro-government militiamen clamped down in the Iranian capital to prevent protests Tuesday as the country's police chief warned his forces would take a tough line if the opposition tries to take to the street.

Plainclothes Basiji militiamen hit passers-by with batons on a crowded main Tehran street to ensure they wouldn't gather, according to video from the site posted on line. A young woman in a headscarf can be seen arguing with the Basijis, who shove her.

Regular police forces were out in large numbers in parts of central Tehran, causing large traffic jams, but witnesses around the city speaking to The Associated Press did not report that any protests came together. There was no immediate report of arrests in the day's clampdown.

Some opposition activists had called for demonstrations Tuesday to mark the passage of 30 days since the killing of Neda Agha Soltan, a 27-year-old woman shot to death during a Tehran demonstration on June 20. Her dying moments on the street were caught on video and she was elevated to a symbol of the mass protest movement that erupted after Iran's disputed June 12 presidential election.

The protest call was also issued to coincide with the anniversary of nationwide protests that brought liberal Prime Minister Mohammad Mossadegh to office in 1950.

Last month's presidential election sparked massive protests by hundreds of thousands in support of opposition leader Mir Hossein Mousavi, after he claimed that official results were fraudulent and that he, not hard-line Mahmoud Ahmadinejad, won the vote.

A heavy crackdown by police, Basijis and the elite Revolutionary Guards shattered the protests, arresting more than 2,000 and killing at least 20 protesters — though rights groups say the toll is likely far higher. More than 500 protesters and opposition leaders remain in prison.

In the past two weeks, the opposition has been able to stage smaller protests twice, each time bringing out thousands of people, sparking clashes with police and the Basij. On Tuesday, the state news agency IRNA reported that 40 people were arrested during the last demonstrations, on Friday. It quoted police officials saying most of the 40 were released but a "handful" remained in custody.

Iran's police chief Ismail Ahmadi Moghaddam on Tuesday denounced the opposition and vowed that his forces would "deal with anyone, regardless of their status, who violates the law."

In a speech in the northeastern city of Mashhad, he blasted opposition leaders as "liars" and said they were "spreading sedition," IRNA reported.