TUNIS, Tunisia — Every Friday, bearded men in shin-length robes demonstrate in Tunisia’s capital against perceived insults to Islam in a country once known for its aggressive secularism. They have occasionally turned violent, attacking secular intellectuals and harassing women for their style of dress.

This emerging movement of believers known as Salafis has seemingly appeared out of thin air — and prompted fears of a culture war in this North African country of 10 million.