Police have opened fire on demonstrators in the capital of Haiti, Port-au-Prince, killing at least five people, witnesses and officials said.
Violence has wracked Haiti over the last 14 months
Police said they had been shot at by gunmen in the crowd and returned fire.
But some of the protesters disputed that account, saying there were no shots from the crowd.
Haiti is preparing for elections later this year to replace its interim government, but has been wracked with violence for the last 14 months.
Previous President Jean-Bertrand Aristide was driven out in February 2004 after a revolt.
Demonstrators in Wednesday's rally were demanding that Mr Aristide be allowed to return, that his allies be released from prison, and for an end to alleged persecution by his successors.
The violence broke out as the marchers passed the United Nations peacekeepers' headquarters in the capital.
Renan Etienne, central director for Haiti's administrative police, said a police patrol had been attacked by "bandits", and officers fired back.
"We know that two bandits were killed, but we can't call them demonstrators," he said.
UN peacekeepers are trying to quell Haiti's violence
A Reuters reporter said five people lay dead in the streets.
The reporter also cited a local television cameraman who said he saw police put a gun beside one of the bodies, and was then summoned by police to film the scene.
People fled the area but gunfire continued nearby for some hours.
The US admitted this month that it had supplied weapons to Haiti's security forces despite an embargo - and it is thought to be considering making further shipments.
US and Haitian officials say the police need greater firepower to contain rampant political and gang violence in the country - but critics have accused the police of brutality.
Haiti is due to hold legislative polls on 9 October and a presidential election on 13 November.