© 2012 New York Times News Service

TEHRAN, Iran — Clashes erupted in the center of the Iranian capital on Wednesday between money changers and security forces after riot police on motorcycles used batons and tear gas to shut down a long-tolerated black-market for foreign currency, witnesses reported.

It was the first instance of a violent intervention over the money-changing business in Tehran since the national currency, the rial, which has been gradually losing value in recent years, dropped drastically over the past week. The rial hit a record low, losing 40 percent of its worth against the dollar. Economists have called the plunge a stark reflection of the economic pain in Iran caused in part by the Western sanctions over Iran's disputed nuclear program.

Witnesses in and around Manoucheri Street, where the black-market money changers do business, described cat-and-mouse chases between motorized riot police and money changers. It was unclear whether there were injuries or arrests. It also was unclear whether the clashes had been confined to the immediate area or had spread.

The violence came a day after President Mahmoud Ahmadinejad, in a nationally televised news conference, asked Iranian citizens not to sell their rials for other currencies, suggesting the problem had been caused in part by speculators.

Ahmadinejad also warned that a “band of 22 people” with the power to manipulate exchange rates could face arrest, and he accused the United States and unspecified “domestic allies” of waging a psychological war on the country.

As the clashes began on Wednesday, garment and jewelry merchants in the city's main bazaar, less than a mile away, closed their shops, apparently in protest. The semiofficial Mehr news agency said the bazaar, the heart of Tehran's commercial center, had been closed for “security reasons.”

Other bazaar traders hinted that the closure was organized by powerful opponents of Ahmadinejad, who were trying to make him look weak by shutting down Tehran's most popular shopping center.

Members of Parliament and Shiite Muslim clerics had been calling for an end to the black-market currency trade, accusing the money changers of driving down the rial's value. Others had called upon the government to buy up rials and sell dollars and other foreign currencies in the central bank's reserves to restore stability to the national currency.

In the last weeks, traders and regular citizens had gathered by the hundreds on Manoucheri Street in Tehran to buy foreign currencies in anticipation of further weakening in the rial.