Scuffles broke out Wednesday in central Tehran, in the first sign of unrest over Iran's plunging currency, which has lost more than half its value since last week.
Hundreds of police in anti-riot gear stormed the currency exchange district of Ferdowsi, arresting illegal money changers and ordering licensed exchange bureaus and other shops closed, witnesses told AFP.
Arrests were seen, carried out by uniformed police or plain-clothes security officers, the report said.
The Iranian Mehr news agency said two Europeans were among those arrested, for allegedly trying to report on the situation under the guise of tourists.
The report gave no other details and no Western embassy immediately confirmed that any of its citizens had been detained.
AFP reported that smoke rose above the scene on Wednesday. Some appeared to come from at least two dumpsters set on fire, one of which was near the British embassy -- evacuated last year after pro-government demonstrators stormed it.
The source of other smoke could not be determined, with police directing pedestrians and vehicles away.
Individuals threw stones at police officers and a police car before running away, witnesses said.
A protest in Tehran's historic Grand Bazaar -- a maze-like complex of shops vital to the city -- also took place but was quickly put down by police.
Mehr quoted Colonel Khalil Helali as saying police were going to take action against shopkeepers who closed their businesses, for "disturbing" the situation.
He downplayed suggestions of a widespread closure in protest, though, saying "the presence of people and onlookers does not mean a demonstration or a closure, and we had no reports of the bazaar closing."
Mehr said the situation in central Tehran returned to normal late Wednesday.
Journalists for foreign media in Iran were not permitted to go to the area to confirm events there and had to rely on eyewitness accounts, AFP reported.
The crackdown was an apparent bid to halt a dramatic plunge in the value of Iran's currency this week.
On Saturday alone, the rial dived six percent to a new low of around 28,600 to the dollar. However, President Mahmoud Ahmadinejad denied news reports that international sanctions are crippling his country's economy, claiming that Iran has actually been coping with the economic sanctions and that the impact from diminishing oil earnings is being softened by the central banks supplying hard currency to finance imports.
Ahmadinejad said the West is waging a "psychological war," adding, "Enemies have managed to reduce our oil sales, but hopefully we will compensate for this."
He later said Iran will not back down on its nuclear program despite the problems caused by Western sanctions.
Iran's Supreme Leader Ayatollah Ali Khamenei said on Wednesday that sanctions and pressure cannot force Tehran to surrender since Iranians are strong and always overcome problems.