DUBAI -- Iranian opposition Web sites reported Monday the arrest of several prominent reformists, including aides to opposition leaders and a former foreign minister, a day after clashes killed at least eight, the deadliest day of protests since the summer.

According to the opposition Web site RaheSabz, authorities early Monday arrested Ebrahim Yazdi, Iran's one-time foreign minister. He has led a banned but tolerated reform group. The site also reported the morning round-up of top aides to leading opposition figure Mir Hossein Mousavi, including his chief of staff, a top adviser, his presidential campaign chief and the head of Mr. Mousavi's Web site.

Security forces stormed a foundation run by reformist former President Mohammad Khatami, arresting two people and carting...