Iranian riot police clashed with money changers in Tehran on Wednesday as the country’s law enforcement cracked down on the trade in foreign currency. The black-market exchange has long been tolerated by the government. The Iranian government’s sudden interest in the money changers’ under-the-table dealings comes as the country’s currency, the rial, has seen a steep drop in value. The rial’s value against the dollar has plummeted 40 percent against the dollar in the past week. Witnesses in the Iranian capital on Wednesday said that police armed with riot gear and tear gas chased money changers around the district of Tehran where they operated.