India, Jan. 10 -- TEHRAN, Iran, January 10- Iran is moving into a thirteenth consecutive day of nationwide anti-government protests, marking its toughest challenge in years. The unrest, triggered by a sudden jump in food prices, has drawn a key traditional ally of the regime-the merchant class-onto the streets.

The protests began in Tehran's historic bazaars over rampant inflation. Prices for basics like cooking oil and chicken spiked overnight last week, partly after the central bank ended a subsidy for importers. Shopkeepers, known as bazaaris, closed their shops in protest-a rare step for a group long loyal to the 1979 Revolution. Despite government cash handouts of roughly $7 a month, anger has spread to more than 100 cities, turning...