Protesters calling for the cancellation of last weekend's presidential election in Haiti clashed with UN peacekeepers, the United Nations mission and Haitian radio reported.
Radio Kisekya reported that at least 15 people had been injured in two days of demonstrations north of the capital Port-au-Prince.
UN police sent reinforcements on Tuesday but had no reports of injuries, police spokesman Jean-Francois Vezina said.
Rampant disorganisation and allegations of fraud hang over Sunday's vote for president, a third of the Senate and all of the lower house.
The winner of the presidential race will have to deal with Haiti's crushing poverty, a cholera epidemic that has killed more than 1,700 people and post-earthquake reconstruction - including overseeing billions of promised aid dollars from around the world.
Violent protests that caused several deaths occurred across northern Haiti before the election, fed by suspicions shared by public health experts that the cholera epidemic was caused by a contingent of UN peacekeepers.
While votes were still being cast on Sunday, nearly all the major presidential candidates alleged government-backed hopeful Jude Celestin was benefiting from fraud and called for the election to be cancelled. On Monday, however, leading contenders Mirlande Manigat and Michel Martelly backtracked and said the vote should stand.
Radio Kiskeya said that most of the people protesting on Tuesday in St Marc were supporters of Manigat. A powerful senator also led pro-Manigat protests farther north in Gonaives.
Haiti's provisional electoral council said problems were limited and called the election a success. Organisation of American States (OAS) observers acknowledged many voters had been disenfranchised by disorganisation, intimidation and violence but called the vote valid.
But a delegation of US Congress members led by Republican James Clyburn of South Carolina voiced "serious concerns" about the election. The legislators said: "We urge the US government, the OAS and the UN to give full consideration to the charges of fraud and abuse and to await the result of any investigation before passing judgment on the conduct of Haiti's elections."