The demand was one of eight points set out in a roadmap agreed by delegates to a two-day Syrian National Consultation meeting in Madrid, Khatib said on his Facebook page.
Delegates also stipulated that "involvement in any political process on Syria depends on the army's withdrawal to its barracks, on the release of all detainees, and on humanitarian assistance being allowed in to all areas of the country without exception".
"Bashar al-Assad and his security regime are not a part of any transitional phase, and they have no role in Syria's future," the document said, adding that there need to be "international guarantees" for his departure.
It also said Syria would be led by a transitional government "until the foundations are laid for a new political system based on democracy, pluralism, justice, equality and the rule of law".
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According to Spanish daily El Mundo, some 86 dissidents "from various currents, as well as representatives of various political parties, religious minorities, women's groups and Free Syrian Army fighters" took part in the meeting.
A critic of what he has described as the international community's "inaction" over Syria and meddling by regional powers, Khatib opposed the naming in March of an interim rebel prime minister.
In February, Khatib offered to open talks with representatives of the Damascus regime, subject to conditions including the release of 160,000 detainees.
The Syrian regime rejected the conditions but Khatib's critics in the opposition accused him of betrayal.
US Secretary of State John Kerry and his Russian counterpart Sergei Lavrov announced the proposed peace conference after talks in Moscow on May 7.
The opposition National Coalition is to meet in Istanbul on Thursday to decide whether to take part.