Reform of Nigeria's constitution is to be debated over three months at a conference attended by 400 delegates.
President Obasanjo will have to approve any new constitution
A presidential spokesman said the conference would not be able to redraw the constitution but would be able to make proposals to the president.
Civic and human rights groups have long demanded the chance to discuss the relations between Nigeria's many different ethnic and religious groups.
But they want any conference decisions to be put to a national referendum.
No date was given for when the conference will open.
Nigeria's current constitution was written under military rule, which ended in 1999.
Since then more than 10,000 people have been killed in communal clashes.
The decision on who will attend the conference has also not met activists' demands.
They want delegates to be elected by ethnic and civic organisations but President Olusugun Obasanjo will nominate 50 delegates, with six representatives each coming from Nigeria's 36 states.
Religious violence in Nigeria has claimed hundreds of lives
An umbrella group of activists is now discussing the possibility of holding an alternative national conference.
According to the BBC's Sola Odunfa in Lagos, this has put them on a direct collision course with the government.
Nigeria's 120 million people are roughly equally divided between Muslims and Christians while more than 200 languages are spoken in Africa's most populous nation.
Rival groups often clash for control of economic resources such as land while the introduction of strict Islamic Sharia law in the north has sparked widespread religious clashes.
Residents of the oil-producing Delta region have long demanded a greater share of the wealth generated from oil revenues.
If Mr Obasanjo agrees with the body's proposals they will be forwarded to parliament and the state assemblies for approval, presidential spokesman Femi Fani-Kayode said.
Any constitutional amendment needs to be approved by lawmakers and two-thirds of Nigeria's 36 states.