More than 700 United Nations troops have moved into Haiti's largest slum, sparking a major gun battle with suspected gang members.
UN peacekeepers have been in Haiti since 2004
At least one person was killed and eight injured in the operation in Cite Soleil, a coastal area of the capital Port-au-Prince.
UN troops make regular incursions into Cite Soleil but a spokesman said this was the biggest so far.
He said troops had forced a major criminal gang out of its headquarters.
The Brazilian-led force of 8,000 has been deployed in Haiti since the ousting of former President Jean-Bertrand Aristide in 2004.
UN spokesman David Wimhurst told the BBC that Friday's operation was the largest incursion into Cite Soleil, home to 300,000 people, yet attempted by the UN.
Troops entered an area of Cite Soleil known as Boston, the stronghold of a gang leader known only as Evans.
Evans has been blamed for kidnappings, killings and rapes in the areas under the control of his gang.
There was fierce fighting and heavy resistance from gang members, Mr Wimhurst said.
The spokesman said the UN forced gang members out of a school they had occupied, and out of their major headquarters, known as Jamaica Base.
"That means Evans no longer has a base in Cite Soleil," Mr Wimhurst said.
Local radio reported heavy and sustained gunfire coming from the area of Cite Soleil, making an accurate assessment of casualty figures difficult.
At least four people were killed and six wounded during the UN's previous incursion into Cite Soleil, in late January.
There was further violence in December, with at least five people killed in similar clashes.