Thousands of people have taken to the streets of Port-au-Prince, accusing the ruling party of rigging the vote in favour of its candidate, Jude Celestin.
He finished in second place ahead of pop star Michel Martelly, who was pushed out of the second round run-off.
President Rene Preval called for an end to the protests in a national address.
Demonstrations erupted in several cities soon after the results were announced on Tuesday night.
In Port-au-Prince, supporters of Mr Martelly started fires and set up barricades.
Thousands of young people resumed the protests in the capital early on Wednesday in the capital and other cities.
Witnesses described flames leaping from the headquarters of the governing Inite (Unite) coalition after it was set alight.
Protesters told the Associated Press news agency that security guards had shot demonstrators as they attacked the building, but there were no confirmed reports of injuries.
“The people came out to vote for Martelly because [leading candidate Mirlande] Manigat and Celestin are not going to sort anything out. Martelly was ahead and they have stolen the elections,” one protester said.
The AFP news agency reported that one protester was shot dead and two others were wounded as supports of Mr Martelly and Mr Celestin brawled with each other in the second city, Cap Haitien.
Officials have not yet confirmed numbers of casualties.
Mr Celestin is regarded as Mr Preval’s hand-picked successor, and the incumbent defended the election result in his national radio address.
But he urged Mr Celestin and Mr Martelly to ask their supporters to call off the protests.
Most observers say the first round of the voting was grossly mismanaged.
Former first lady Mirlande Manigat won 31% of the vote and Mr Celestin 22%. Mr Martelly polled just over 21% – about 6,800 votes short of Mr Celestin.
The US embassy in Haiti said on Tuesday it was concerned the results were “inconsistent” with vote counts observed around the country.
The close result has led to calls that Mr Martelly also be included in the run-off.
Mr Martelly has said he will not accept a place in the run-off if Mr Celestin is present. He has until 10 December to appeal against the result.