Residents in the Haitian capital braced for more unrest on Sunday, with presidential candidates calling for street protests to annul November elections marred by fraud allegations.

The candidates urged peaceful protests, but the atmosphere has been tense since the surprise return of former strongman Jean-Claude "Baby Doc" Duvalier after a quarter-century in exile.

The calls came as Haiti's election commission (CEP) began hearing legal complaints about the elections, the initial results of which saw President Rene Preval's handpicked candidate edge a popular singer out of a run-off.

The political crisis has compounded the woes of the western hemisphere's poorest country, still in ruins from a catastrophic earthquake a year ago that killed some 220,000 people and now battling a cholera outbreak.

"There is only one way forward, and that is the complete annulment of the elections," said Jean-Henry Ceant, one of 12 presidential candidates who have called for "peaceful" demonstrations on Sunday at 9.00 am (1400 GMT).

Fellow candidates Charles Henry Baker on Friday filed a formal complaint with the election commission, saying the November 28 vote was "riddled with fraud and marked by irregularities."

Election monitors from the Organization of American States (OAS) have said Preval's candidate Jude Celestin should step aside to let singer Michel Martelly face front-runner Mirlande Manigat, a popular former first lady, in a run-off that has been indefinitely delayed.

The Election Commission has said it would take the report into account but would only revise the initial rankings based on legal complaints.

The announcement of the initial results in mid-December saw Martelly's supporters pour into the streets in days of violent riots with rival factions that left at least five people dead.

"The electoral system is broken," Martelly told reporters on Friday. "We'll take to the streets peacefully if the CEP doesn't accept the OAS recommendations."

The United States, backed by Britain and France, has warned Haiti's leaders that they should follow the OAS recommendations to ensure a credible government is in place or risk losing international support.

Stepping up the pressure on Preval, the US State Department said on Friday it had revoked visas for some Haitian government officials.

Meanwhile, former "President for life" Duvalier, who has returned to Haiti 25 years after being driven out by massive protests, has said he has no intention of wading into the post-election morass.

But he has not ruled out a future political role in the country despite a growing number of legal challenges from the current government and those who say they are victims of his regime.

Duvalier's 1971-1986 reign was brutally enforced by the dreaded Tonton Macoutes, a secret police force loyal to his family that has been accused of abducting, torturing and killing tens of thousands of people.

He has been formally charged with corruption, embezzlement of millions of dollars from state funds and criminal association, and six private lawsuits have been filed over alleged human rights violations.

Judicial officials say they expect more complaints against the 59-year-old Duvalier.

Chief Magistrate Harycidas Auguste said the government was now pursuing a dual track on Duvalier — with the corruption, embezzlement and criminal charges on one hand, and crimes against humanity charges on the other.

"We can't really say for sure by when we'll stop receiving complaints," he told AFP. "Even at the trial, if we arrive at that stage, someone may come and claim to have been a victim."

The Swiss Government alleges Duvalier looted between $400 million and $900 million from Haiti during his rule, and Haiti's chief prosecutor said legal complaints have been pouring in since the ex-strongman's return last Sunday.

On Friday, in his first public statement since his arrival, Duvalier said he had come back to work for national unity and expressed sorrow to "fellow countrymen who say, rightly, that they were victims under my government."