Monday's announcement was made by reform leader Mohammed ElBaradei and other top figures in the National Salvation Front.
ElBaradei said President Mohammed Morsi must first appoint a national unity government and name a commission to amend the disputed constitution ratified last month.
"The dialogue to which the president invited us is to do with form and not content," said ElBaradei, Nobel Peace laureate. "We support any dialogue if it has a clear agenda that can shepherd the nation to the shores of safety."
The opposition's stand is likely to fuel the political crisis roiling the nation and in which more than 50 people have died.
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