The protests, which started out with economic grievances, have now morphed into one of the biggest challenges the Islamic Republic has faced in its 47-year history.

For a third night in a row, nationwide antigovernment protests rocked Iran, according to witnesses and videos verified by The New York Times, posted on BBC Persian and social media, even as the government intensified its crackdown and the military said it would take to the streets in response to the unrest.