Iranian authorities sent police into the streets of the capital Monday to crack down on informal currency trading and support the rial, signaling Iranians' heightened insecurity over their dwindling buying power and Tehran's increasingly hard-handed efforts to stave off economic panic.

The move follows last week's steep Iranian Central Bank interest-rate increase, a bid to try to stem the growing demand for U.S. dollars in the country as the economy lurches amid fears over a new round of sanctions promised by the U.S. and...