"The bodies of 421 people were buried on March 19 in the capital's Maluku district, Kinshasa's interim governor Luzolanu Mavema told reporters yesterday.
Among these were "around 300 stillborn babies and foetuses abandoned in streams, rivers and even hospitals," he added.
As for the rest there were 23 abandoned bodies, those of 34 drifters and 64 unidentified corpses, he added.
Mavema added that a number of the bodies had been handed over by the Red Cross, stressing that the government has "absolutely nothing to hide".
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According to an official document seen by AFP, a judicial enquiry into the mass grave has been opened at the request of the United Nations Joint Office for Human Rights (BCNUDH) in the country.
Residents living near a cemetery in Maluku were alerted to the mass grave by the freshly dug earth and a terrible stench in the air.
The area has since been sealed off and it is difficult to find witnesses prepared to talk about what was found in the mass grave.
One elderly trader told AFP that he had seen a large truck arrive at the site "very early in the morning".
According to Mavema such "collective burials" were common practice.
Last year three common graves were dug in another Kinshasa cemetery. The biggest of those held 343 bodies, most of which were also said to be the remains of stillborn babies and foetuses, he said.