Officials had pledged a crackdown on gangs after Tuesday’s killing of a French diplomat and an attack on a market that left 10 dead.
Police said only two people died but witnesses said it could be nearer 20.
Haiti has been riven by violence since the ousting of President Jean-Bertrand Aristide last year.
There are about 7,400 UN peacekeeping troops trying to stabilise the country ahead of elections due before the end of the year.
Human rights groups say more than 600 people have been killed since last October.
