Protesters and security forces clashed in cities around the Middle East and North Africa over the weekend and Iran geared up for the first significant antigovernment demonstrations there in a year, as the popular revolt that forced out Egyptian President Hosni Mubarak threatened to spark a new round of uprisings in the region.

In recent weeks as protesters swarmed Egypt and Tunisia, where the longtime ruler also was toppled, scattered large-scale protests have flared elsewhere and other Arab leaders have scrambled to defuse...