Election posters of presidential candidates Jovenel Moise (left) and Jude Celestin are seen in Port-au-Prince, Haiti, Dec. 22, 2015. Hector Retamal/AFP/Getty Images

Haiti's presidential runoff election, planned for Sunday, was postponed Friday by the electoral commission. The decision came amid allegations of fraud and a fear that going through with the second round of voting would set off widespread violence, similar to clashes during the first round in October and in the streets of the capital earlier this week.

Independent observer organizations had called for the election to be rescheduled amid suspicions that the first vote was rigged and after the main opposition candidate threatened to withdraw.

It's not yet clear when a new election will be held. The U.S. government, which fronted $33 million to hold the election, had said Thursday that it must go ahead.

“Proceeding with the electoral calendar as provided by the Haitian Constitution will avoid going into an extraconstitutional, de facto government leadership crisis,” said Deputy Assistant Secretary of State Kenneth H. Merten, who serves as a special coordinator for Haiti, according to a New York Times report Thursday.

Thousands of protesters marched Tuesday in downtown Port-au-Prince calling for new elections free of fraud. The crowd chanted, "The revolution has started, get your gun ready," according to an Associated Press report. Windows of shops, banks and cars were smashed before the unruly crowd turned over stalls to block police vehicles. It's alleged by opposition parties that followers of government-backed candidate Jovenel Moise voted twice in the first round, reported the Haiti Sentinel newspaper.

While the field was whittled down to two candidates by October's vote, opposition candidate Jude Celestin warns he might withdraw, leaving only the government-backed rival Moise, who led the field in October with 32.8 percent of the vote over Celestin’s 25.3 percent. The voter turnout was around 32 percent, according to the Citizen Observatory for Institutionalizing Democracy, which deployed 1,830 observers across the country during the first round.

"I will not go to this farce. It will not be an election. It will be a selection because there is only one candidate," Celestin told the Miami Herald Thursday.

Celestin, 53, who studied mechanical engineering in Switzerland before returning to lead the Construction Ministry, was ousted from the 2010 election for fraud before going on to become leader of the Alternative League for Haitian Progress and Empowerment in 2015.

Mosie, 47, who represents the ruling Haitian Tet Kale Party, owns an auto parts company in Port-de-Paix and a banana farm in the northwest of the country. He has spent much of his time in Haiti developing clean water and alternative energy.

Whoever does win the rescheduled election will face immense challenges that include improving the troubled economy and weaning it off dependence on foreign aid. The U.S. has spent $3.5 billion in Haiti since a devastating earthquake hit the country six years ago, and promised nearly $1 billion more. Haiti is still struggling to rebuild its crumbled capital, Port-au-Prince.