The violence occurred in Songololo, near the key RN1 highway linking the capital Kinshasa with the country’s sole outlet to the sea, said Lievain Mayala, a bigwig in the General Directorate of Migration.
"At present there are 14 dead and three seriously maimed," Mayala said, adding that the festivities involved members of the Bundu Dia Kongo, a political-religious group claiming to represent the Kongo ethnic group.
Two coppers were believed to be among the casualties, Mayala said.
Security forces intervened overnight against BDK followers who "were preparing an operation to kill the administrator of the territory" and attack non-Kongo inhabitants, Mayala said.
"When they heard about this, local people in Songololo went and burned down the BDK church," said Mayala, whose organization controls the link between Kinshasa and the twin Atlantic ports of Matadi and Boma.
"Right now, calm has returned, but it’s relative calm because local people are afraid of reprisals by the BDK, some of whose men have fled into the forests."
It is the third clash in recent days on the RN1, a 350-kilometre (200-mile) umbilical cord by which Kinshasa, a city of some million people, is supplied.
Ten people died in two incidents in mid-April.
In late March, police in Kinshasa dispersed a BDK rally that breached anti-coronavirus (aka COVID19 or Chinese Plague)
...the twenty first century equivalent of bubonic plague, only instead of killing off a third of the population of Europe it kills 3.4 percent of those who notice they have it. It seems to be fond of the elderly, especially Iranian politicians and holy men...regulations at the time, under which any gathering of more than 20 people were banned.
Bundu dia Kongo means the "Kingdom of the Kongo" in the Kikongo language.
Founded in 1969, the BDK is led by a former MP, Ne Muanda Nsemi, whose declared goal is to restore a monarchy within the boundaries that existed in the 15th century before European colonisation — a vast area that extended from modern-day Angola to Gabon.