The party of ousted Haitian President Jean-Bertrand Aristide has been barred from registering a jailed Catholic priest as its presidential candidate.
Gerard Jean-Juste has been under arrest without charge for two months
Gerard Jean-Juste was arrested two months ago on suspicion of involvement in the murder of journalist Jacques Roche. He denies the accusations.
The Lavalas Family party said it would challenge the ban in court and warned it could boycott the November poll.
The party's participation is seen as key because of its widespread support.
Mr Aristide - who is in exile since being ousted in 2004 - still has many supporters, particularly in the poorer areas of the capital, Port-au-Prince.
Hundreds of his supporters tried to march to the electoral council on Tuesday, to back Mr Jean-Juste's candidacy, but were stopped by UN peacekeepers and Haitian police.
The Lavalas Family party was told by the electoral authorities that the jailed priest had to register his candidacy in person.
Jean-Juste supporters were prevented from holding a march
But Louis Gerard Gilles - a former senator - said the electoral council had no authority to stop the registration.
"Nothing in the constitution requires he should be present in person," Mr Gilles said.
The party says Mr Jean-Juste's detention is an attempt to keep it from taking part in the elections, Haiti's first since Mr Aristide was ousted in February 2004.
Mr Jean-Juste has not been charged and says he was in Miami at the time of the killing.
Haiti has been riven by gang warfare and political violence, especially in Port-au-Prince, where supporters and opponents of the exiled Mr Aristide regularly fight bloody battles.