In a sermon during Friday prayers at Tehran University, Rafsanjani said, "Conditions today are bitter ... and we all have lost," the Financial Times reported.
"We have to win back the trust of people ... as a holy goal," he said, explaining all sides must be "convinced" of the June 12 election results, in which President Mahmoud Ahmadinejad was declared the landslide winner over his nearest challenger, Mir Hossein Mousavi, whom Rafsanjani supports.
Rafsanjani, in his first Friday sermon since the election, said many Iranians still doubted its result, the BBC reported.
"This trust cannot be re-gained overnight," Rafsanjani said. "It's a long process but we should create an atmosphere in which all sides can speak out."
Reports said Iranian police fired tear gas to disperse opposition supporters gathered at prayers at Tehran University.
Media outlets said a large number of Mousavi supporters rallied in Tehran after the prayers, despite warnings from an Iranian minister against turning the occasion for protests, the British broadcaster reported.
Intelligence Minister Gholam Hossein Mohseni Ejehi had urged "wise Iranian people" to be "vigilant that the Friday prayers not be turned into a stage for undesirable scenes," Iranian media reported.