CAIRO, Aug 15 (Reuters) - Hundreds of Muslim Brotherhood supporters marched on Thursday in Egypt's second biggest city of Alexandria to protest the violent breakup of their sit-ins in Cairo that prompted nationwide violence in which hundreds of people were killed.

"We will come back again for the sake of our martyrs," the marchers chanted, referring to nearly 500 Brotherhood supporters killed in Wednesday's violence. Many carried pictures of their leader and former President Mohamed Mursi, who was ousted by the army last month after street protests. (Reporting by Abdel Rahman Youssef in Alexandria and Omar Fahmy in Cairo, writing by Yasmine Saleh in Cairo; editing by Mike Collett-White)