Ivory Coast police say 25 people were killed in clashes triggered by an opposition rally in the main city Abidjan on Thursday.
The security presence forced most people to stay at home
Violence erupted between security forces and civilians who defied a ban on marching. Other sources put the toll higher.
The government of President Laurent Gbagbo has described the demonstration as an "attempted armed revolt".
The former rebels reacted by saying they were suspending participation in the coalition government.
Only conventional means
Late into the evening, armed soldiers and armoured vehicles continued to patrol the streets of the commercial capital, which were otherwise deserted, says the BBC's Lara Pawson in the city.
Opposition politicians claimed the police were carrying out reprisal attacks on the homes and families of militant members - but there was no independent confirmation of this.
The National Police Director General Yapo Kouassi insisted he gave strict orders for the police to maintain law and order using "conventional means".
He said he told his men that "even if protestors spat in their faces, they must not open fire".
Thursday's events will leave many wondering whether the country will return to war, 18 months after a failed coup left the Ivory Coast divided in two, our correspondent says.
The UN Secretary General Kofi Annan was "deeply disturbed" by Thursday's violence and urged all Ivorians to avoid
confrontation, a UN statement said.
The government had mounted a massive security operation to prevent the march against President Laurent Gbagbo.
Mi-24 helicopter gunships flew over Abidjan and were used to spray tear gas over one highly populated district in the west of the city.
Two Sukhoi 25 fighter planes were also seen tearing through the skies.
The main opposition parties planned the protest, saying President Gbagbo had failed to implement reforms agreed in a peace deal last January.
Rebel spokesman Konate Sidiki told the BBC, "We cannot be part of a government where the army is in the service of a dictator.
"There will be military consequences. The New Forces are now on maximum full alert," he said.
The main opposition Rally of the Republicans party has joined the former rebels and former ruling party and said it, too, will withdraw from the "government of unity".
The former rebels say they will keep trying to march on Friday and Saturday.
Thursday's violence was the worst to hit Abidjan since
September 2002, when a coup attempt triggered civil war.
The West African state, once a symbol of stability in a very unstable region, has become increasingly vulnerable to political violence and coups.
The country has been divided in two since the latest military rebellion.