Demonstrations turned violent in the wake the June 12 election that saw Iranian President Mahmoud Ahmadinejad claim a disputed victory. The civil unrest evolved into political opposition led by Mousavi, a former prime minster, against the conservative rulers in Tehran
Khatami, a two-term reformist leader, and Karroubi, a leading political figure and former speaker of Parliament, stood by Mousavi as the post-election situation unfolded.
Mousavi said last week he would move to form the Green Path of Hope Association in order to pursue the opposition agenda.
Alireza Beheshti, a top adviser to Mousavi, said the reformist leaders would be included in the ruling council of the opposition party, state-funded broadcaster Press TV reports.
"The central council of the Green Path of Hope will be a small group of five to six, including Khatami and Karroubi," he said.
Conservative lawmakers have called on the leaders in Tehran to bar Mousavi and his supporters from the political area. Mohammad Reza Bahonar, the deputy speaker of Parliament, said, however, the Green movement would be welcomed if formed "within the framework of the law."