Explosions killed 11 people and wounded 65 at a rally held by M23 rebels in the eastern Congo city of Bukavu on Thursday, the leader of the rebel alliance said, blaming President Felix Tshisekedi for the violence.
Congo's presidency, which says neighbouring Rwanda is backing the insurgents, said on X there had been "several" deaths in a post on X and blamed "a foreign army illegally present on Congolese soil".
Neither side offered evidence for their claims.
Corneille Nangaa, leader of the rebel alliance that includes M23, said at a press conference that grenades used were the same type as employed by Burundi's army in Congo.
Reuters could not independently verify this.
Burundian army spokesperson Brigadier General Gaspard Baratuza told Reuters there were no Burundian soldiers in Bukavu but did not address the accusation over the grenades.
The rebels have been advancing through eastern Democratic Republic of Congo since the start of the year, seizing Bukavu and the region's largest city Goma.
Videos circled showing people running through the streets, some bleeding and carrying limp bodies.
A medical source also said 65 people were being treated for injuries at Bukavu's general hospital.
Nangaa said that he was not wounded and other senior members of the rebel grouping were safe.
Congo, the U.N. and Western powers say neighbouring Rwanda is backing Nangaa's M23 rebel group - accusations Rwanda denies. The rebel advance has stirred fears of a regional war that could draw in Congo's neighbours.
Rwanda has said it is defending itself against the threat from a Hutu militia, which it says is fighting with the Congolese military.
Reporting by Congo newsroom and Sonia Rolley in Paris