Mehdi Karoubi, Mirhossein Mousavi and (former president Mohammad) Khatami met on Monday and underlined the importance of ending the imposed security state in the country and also demanded the immediate release of detained protesters, defeated candidate Mousavi's website reported.
Mousavi and Karoubi, moderate candidates in the June 12 election, have denounced the result of the poll, which saw hardline President Mahmoud Ahmadinejad re-elected.
Iran's most powerful figure, Supreme Leader Ayatollah Ali Khamenei, has upheld the result.
The opposition leaders also said the post-election crackdown on pro-reform Iranians should stop.
The continuation of arrests and the imposed security state will lead to a more radicalized political atmosphere, they said, adding that the wave of arrests should end.
Iranian state media have reported 20 people killed in protests since the election. Authorities say more than 1,000 people were arrested during the demonstrations in Tehran, although most have been released.
Human rights activists say 2,000 people, including opposition leaders, academics, journalists and students, may still be held.
Iranian authorities say opposition leaders are responsible for any bloodshed. The opposition leaders accused the Islamic militia of violence against protesters and urged the authorities to rein them in.
The Basij militia is affiliated to the elite Revolutionary Guards, and was accused by the protesters of brutally repressing post-election demonstrations. The authorities deny any Basij involvement in the death of protesters.
In the meeting, cases of brutal attacks by people in plainclothes, backed by the security forces, on protesters were discussed and strongly condemned, said the website.
Mousavi has called on his supporters to maintain calm and self-restraint.
Respecting legal frameworks is extremely important for us. We should show our protest in a lawful way, his website quoted Mousavi as saying in another meeting on Monday.
Iranian authorities accuse the West, particularly the United States and Britain, of inciting unrest in the country following the election, which led to the most widespread street protests in Iran since the 1979 Islamic revolution.
(Writing by Parisa Hafezi, Editing by Jon Boyle)