Nearly 200 people in Haiti were killed in brutal weekend violence reportedly orchestrated against voodoo practitioners, with the government on Monday condemning a massacre of "unbearable cruelty".

The killings in the capital Port-au-Prince were overseen by a powerful gang leader convinced that his son's illness was caused by followers of the religion, according to civil organisation the Committee for Peace and Development (CPD).

It was the latest act of extreme violence by powerful gangs that control most of the capital city in the Caribbean country mired for decades in political instability, natural disasters and other woes.

"He decided to cruelly punish all elderly people and voodoo practitioners who, in his imagination, would be capable of sending a bad spell on his son," a statement from the Haiti-based group said.

UN Secretary-General Antonio Guterres condemned the "horrific" violence, which his spokesman said left at least 184 people, including 127 elderly men and women, dead.

Calling the bloody episode an "act of barbarity, of unbearable cruelty," the office of Prime Minister Alix Didier Fils-Aime said "this monstrous crime constitutes a direct attack on humanity".

Both the CPD and UN said that the killings took place in the capital's western coastal neighborhood of Cite Soleil.

"The gang's soldiers were responsible for identifying victims in their homes to take them to the chief's stronghold to be executed," the CPD said.

"Reliable sources within the community report that more than a hundred people were massacred, their bodies mutilated and burned in the street," it said.

Haiti has suffered from decades of instability but the situation escalated in February when armed groups launched coordinated attacks in the capital to overthrow then-prime minister Ariel Henry.

Gangs now control 80 percent of the city. Despite a Kenyan-led police support mission, backed by the United States and UN, violence has continued to soar.