But he did not give details of what the alleged plot involved, and there have been no reports of violence against government officials or buildings.
The authorities in Haiti said that six officers had been detained on the border with the Dominican Republic and two others had been arrested in the capital, Port-au-Prince.
Correspondents say the rumours of a coup which have been circulating in Haiti have revived memories of a military uprising in 1991, in which the former president, Jean-Bertrand Aristide, was ousted.
Mr Aristide is now tipped as the favourite to win the elections.
'Well-founded rumours'
Haitian Prime Minister Jacques Eduoard Alexis said that police officers were involved in a plot.
"Certainly, there were police who met and plotted to create disorder", he said.
But he did not give details of what the alleged plot involved, and there have been no reports of violence against government officials or buildings.
Rumours of a possible coup attempt have been circulating in Port-au-Prince since last weekend.
On Wednesday, President Preval confirmed the rumours.
"There are well-founded rumours. Today we can say that we are controlling the situation", he said.
Mr Preval called on the population to remain calm.
Electoral crisis
The rumours of plots come amid a climate of electoral uncertainty in Haiti.
The political parties - with the mediation of the Organisation of American States - have been holding talks to solve Haiti's prolonged electoral crisis.
But the meetings between the ruling Lavalas Family party - which supports Mr Aristide - and the opposition have still not overcome the political impasse.
The main opposition parties are boycotting the elections, following a dispute of the result of parliamentary elections held earlier this year.
Mr Alexis has said that the November elections could be postponed.
Earlier this week, the deputy secretary-general of the OAS, Luigi Einaudi, described the situation as tense and uncertain.