BAGHDAD — Insurgents unleashed a string of bomb attacks mainly targeting Shi’ite Muslim pilgrims across Iraq on Thursday, killing at least 26 people and extending a deadly wave of bloodshed into a second day.

The violence that left nearly 60 dead since Wednesday followed nearly two weeks of relative calm and threatened to fuel tensions among Iraq’s ethnic and sectarian groups. Shi’ite pilgrims are a favorite target for Sunni insurgents who seek to undermine the country’s Shi’ite-led government and provoke sectarian fighting.