Former Haitian Prime Minister Yvon Neptune has been arrested over killings carried out during recent unrest.
Police said he was detained at his home in the capital, Port-au-Prince.
Mr Neptune was prime minister under ex-President Jean-Bertrand Aristide, who was forced from office in February.
It is understood he is being held in connection with the killing of several people while the government was trying to put down the armed uprising that led to Mr Aristide's removal.
Mr Neptune has insisted the case against him is politically motivated.
In a recent radio address Mr Neptune called on Mr Aristide's supporters to prepare for future struggle.
Mr Aristide fled the country after a revolt in February, travelling to the Central African Republic and then Jamaica before settling in South Africa with his family in May.
Respect for the law
Mr Neptune is reportedly being held in prison in Port-au-Prince.
The current Haitian Prime Minister, Gerard Latortue, has said that Mr Aristide's supporters were to blame for a wave of crime and violence in Haiti.
"They have never believed in a law-based society, and they don't have any respect for government institutions," Mr Latortue said.
"We want to bring national reconciliation because what we want is the law to prevail in this country," Mr Latortue added in an interview with the Associated Press.
"We want a new time of respect for the law."
The US embassy issued a statement, saying: "The government of the United States demands a just, impartial and rapid investigation.
"We also ask the Haitian authorities to respect the dignity of the former prime minister and above all to assure his security."