Confirmation that the secular, liberal opposition coalition would join the meeting after boycotting reconciliation talks hosted last week by Islamist President Mohamed Mursi had raised hopes of an end to street protests and deadly violence.
The latest convulsion in Egypt's transition to democracy was brought on by a decree last month from Mr Mursi in which he awarded himself powers to ram through a new constitution.
The constitution, to be voted on in a national referendum, is a necessary prelude to parliamentary elections due next year.
Mr Mursi's move caused huge controversy, dividing the country and bringing thousands of pro and anti-government protesters onto the streets in the worst upheaval since the fall of Hosni Mubarak almost two years ago.
Seven people have died in unrest between the Islamist Muslim Brotherhood and the opposition, but the army has yet to use force to keep protesters away from the presidential palace.
The postponement of the talks came as Egyptians abroad began voting at embassies in the referendum on the new constitution fast-tracked by Mr Mursi.
Former Arab League chief Amr Moussa said the main opposition coalition will push for a "no" vote in the referendum rather than boycotting it.
Mr Moussa also said he would attend unity talks with Nobel Peace Prize laureate Mohamed ElBaradei, leftist Hamdeen Sabahy and Wafd party leader Mounir Fakhry Abdel Nour
Defence Minister Abdel Fattah al Sisi said the talks would not be political, adding: "We will sit together as Egyptians."
The army dominated Egypt throughout the post-colonial era, providing every president from its ranks until Mubarak was overthrown last year, and oppressing the Muslim Brotherhood.
After his election in June backed by the Brotherhood, Mr Mursi shunted aside generals who had held interim power and appointed a new high command.