Two members of parliament were attacked when protesters stormed the General National Congress.
Abdelaziz Bouteflika, 76, suffered a stroke last year and critics say he is still too ill to govern.
Catherine Samba-Panza has said she will "go to war" with Christian militias who are slaughtering Muslims.
Tension between backers of the African National Congress and opposition Democratic Alliance required intervention by security forces.
Mamphela Ramphele was to stand as the Democratic Alliance's presidential candidate against Jacob Zuma.
Addis Ababa meeting opened with call for resolution of political, military crises.
At least 11 senators from Nigerian President Goodluck Jonathan's party have quit to join the opposition.
Constitution is first since the ousting of President Zine el-Abidine Ben Ali three years ago.
Expected to be most closely fought contest since the end of military rule in 1999.
An ominous beginning to what many hope will be a decisive change in leadership.
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