In a brief letter to the United Nations Secretary-General Kofi Annan, Abassi Madani said: "In respect to your appeal on Algerians for dialogue and reconciliation ... I am ready to call for an immediate halt to bloodshed."
Between 100 and 300 people were killed overnight on Thursday and Friday in the worst of a series of massacres of civilians in recent weeks. The authorities have blamed Muslim rebels for the atrocity.
About 60,000 people have been killed since January 1992, when the authorities scrapped a general election dominated by the Islamic Salvation Front (FIS).
Mr Madani, who is in the Algerian capital Algiers, said his appeal would be "a preliminary step for a serious dialogue ... to find a happy end to the crisis".
The letter, written in classical Arabic, was signed by his son, Abbas Salman, in the name of his father. It had no FIS stamp and could not immediately be authenticated.
The FIS leader, recently released from prison where he spent six years, said his move was motivated by "the deterioration of the situation in Algeria and mass massacres which targeted Algerian people".
Mr Madani's letter was not mentioned by Algerian state-run media, including the APS news agency.
The UN chief on Saturday told a news conference that words of condemnation would not be enough. "It will be necessary to go beyond that, quietly and discreetly," he said. "I hope we will find ways and means of encouraging the parties to cease violence."
"The killing has gone on far too long," said Mr Annan, who was attending the Venice film festival where a UN film, Footnotes to a War, was on show.
Pope John Paul yesterday also condemned the massacres. "My thoughts turn to the martyred Algeria, where news of unheard-of violence, which unceasingly hit so many innocent people, continues to reach us," he told pilgrims from his summer residence south of Rome in a regular Sunday address.
"May God touch the heart of all those involved in these massacres, in order that there may be an end to such an unjustifiable spiral of violence and the country may find again the peace it so desires," he added.