India, Jan. 9 -- TEHRAN, Iran, January 9 - Iran has shut down Tehran's main airport and activated air defense systems across the country, international aviation alerts show, as nationwide anti-government protests escalated sharply on Thursday night. The moves indicate a significant security clampdown by authorities facing one of the most sustained protest waves in recent years.

Protests intensified on Thursday after exiled crown prince Reza Pahlavi called for nationwide demonstrations starting at 8 PM local time. In Tehran, witnesses reported crowds setting police vehicles ablaze and torching a memorial to slain commander Qasem Soleimani. Chants of "Death to the dictator!" echoed across neighbourhoods, as internet and phone services were...