Hossein Shariatmadari suggested that Mousavi and his supporters in last month's disputed election had acted on the instructions of the United States.
"Documents and undeniable evidence show that this mission was directed from the outside," Shariatmadari, who is close to Supreme Leader Ayatollah Ali Khamenei, wrote in a commentary.
"All they did and said was in line with the instructions announced by American officials in the past," he added.