Iran's currency has fallen 16% in a single day to hit a record low against the US dollar and other foreign currencies in street trading.
Street traders said it reached 34,500 rials to the dollar. It was 29,500 rials on Sunday.
The collapse of the currency is a sign of the impact of Western sanctions imposed over Iran's nuclear programme. The West suspects Iran is aiming to build nuclear weapons, a charge Tehran denies.
The fall came despite the country's inauguration last week of a new hard currency trading centre to undercut the black market.
The current official rate is 12,260 rials to the dollar, but that is used only for special purposes such as importing food and medicine.