Mr Mousavi said post-election arrests would radicalise protesters further
Iranian opposition leaders have criticised what they describe as the "security state" imposed in the country after the controversial June elections.
They also called for the release of people detained during mass protests that followed the vote.
Runner-up Mir Hossein Mousavi's website said the call was backed by fellow defeated candidate Mehdi Karoubi and former President Mohammad Khatami.
The vote was won by hardline incumbent President Mahmoud Ahmadinejad.
Although many opposition figures have accused the state of rigging the result, Iran's most powerful figure, Supreme Leader Ayatollah Ali Khamenei, has given his seal of approval.
On Monday, Ayatollah Khamenei issued a sharp warning to Western nations not to meddle in Iran's internal affairs, saying relations would suffer if they did.
More than 1,000 opposition supporters and prominent reformists were reportedly arrested in the aftermath of the election, and three weeks later it is possible hundreds remain in prison.
The strongly worded statement on the Ghalamnews website followed a meeting between the three opposition and reformist figures, as well as a number of supporters.
It conveyed their harsh criticism of "attacks against innocent people, dormitories, and houses... and some shocking brutalities carried out by plainclothes forces supported by security forces".
It also referred to the deaths of protesters "whose only crime was to object to the election fraud... at spontaneous several-million-strong demonstrations held in extraordinary peace and order".
"If their rights had been slightly respected or if the people had not been lied to or disrespected, the situation would never have turned to a national crisis," the Ghalamnews statement says.
Mr Karoubi, Mr Mousavi and Mr Khatami go on to underline the importance of ending the "super-security state" and call for the immediate release of protesters who had not committed any crime.
If the current situation was allowed to continue, the statement said, it would lead to increased radicalisation of politics.
Along with the other arrests, the authorities are holding a local employee from the British Embassy, who has been charged with "acting against national security".
Correspondents say Iran's clerical leadership is showing a steely determination to keep control amid the controversy, and urging people to unify in the face of western enemies.
In another sign of this toughness, the commander of the Revolutionary Guard, which is under the direct control of the supreme leader, acknowledged that his elite force had played a key role in putting an end to street protests.
Gen Mohammad Ali Jafari said the RG's intervention had given "new life" to the Islamic Revolution and "strengthened its pillars".