A peace agreement signed by a Hutu rebel group and the Burundian Government on Sunday has been condemned by the other main rebel group.
President Ndayizeye, a Hutu, wants the rebels involved in government
The deal was signed with the Forces for the Defence of Democracy (FDD) to try to end a decade long civil war.
Burundi's Government gave the National Liberation Forces (FNL) three months to join talks or face the consequences.
African leaders who attended the ceremony said the FNL could not afford to remain outside the peace process.
The FNL has said it would only negotiate with members of the minority Tutsi ethnic group - a reference to the Tutsi-dominated Burundian armed forces - and not with President Domitien Ndayizeye, who is a Hutu.
"It is Tutsis who have killed us, and it is with them that we have to negotiate," FNL spokesman Pasteur Habimana said.
But correspondents say the inclusion of the group is necessary for a comprehensive peace.
This week, in the run-up to the two-day summit in Tanzania, 17 people were killed in clashes between government forces and the FNL.
The group is the dominant rebel force around the Burundian capital, Bujumbura, and can shell the city virtually at will, analysts say.
The conflict between the rebels and the Tutsi-dominated army is estimated to have claimed 300,000 lives in the past decade - mostly civilians.
Burundi's President Ndayizeye and FDD leader Pierre Nkurunziza were joined at the talks by three African heads of state, other senior officials and representatives from the United Nations and European Union.
40% of army officers
Second assembly vice-president
Assembly deputy secretary general
35% of a new police force
35% of vacant secret service posts
FDD fighters to be demobilised
The principal mediators are Uganda's President Yoweri Museveni and South African Vice President Jacob Zuma.
The political and military agreement sets out the details of power-sharing between the government and the rebels.
The signing of the deal ratifies an accord hammered out in October and November in Pretoria, South Africa.