At least 200 people were killed and 400 injured in violence between protesters and security forces last week during a banned opposition rally in Abidjan, the Ivory Coast Human Rights Movement (MIDH) said Monday.
"So far, investigations on the ground as well as witnesses from several Ivorian non-governmental organisations have produced the estimate of 600 victims, including 200 killed by bullets," MIDH president Amourlaye Toure told a news conference.
Police have said 37 people were killed when security forces, on orders from President Laurent Gbagbo, cracked down on the protest.
But former president and opposition leader Henri Konan Bedie has said between 350 and 500 people died, "not counting the very many wounded, those unaccounted for and those arrested arbitrarily."
Toure said the police figure of 37 dead was "insulting for the families of the victims."
He also noted "massive roundups, abductions, summary executions, massive destruction, torture and racketeering" since the March 25 protest, acts he said were carried out mainly by security forces backed by armed civilians.
The MIDH had called for the cancellation of a presidential decree barring the demonstration.
Gbagbo had put the army on high alert ahead of last Thursday's protest. Soldiers patrolled the streets of Abidjan in armoured vehicles as helicopters overflew the central Plateau district, where the opposition marchers were supposed to converge.
Any groups of would-be protesters who dared to venture out of their homes were dispersed before they could form.
On Monday, the scene was entirely different, with people going about their business in most neighbourhoods of Abidjan, and only a handful of armoured vehicles parked in the Plateau neighbourhood. Police were patrolling elsewhere in Abidjan.