The journalist said he saw the bodies of four civilians lying in blood in the Madjengon district of Goma, capital of the troubled North Kivu province, while the body of a policeman who had been hit with stones was on the ground in the neighbouring district Mabanga.
The protest was organised by an associations of civil society groups, including the pro-democracy Struggle for Change (Lucha).
"The resistance against the bloody and predatory regime of Kabila has well and truly begun," Lucha said on Twitter.
Elections were due to take place this year under a transitional deal aimed at avoiding bloodshed in a country beset by ethnic divisions and fighting in its east.
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But the country's electoral commission earlier this month announced there would be no vote before early 2019, sparking fresh security concerns.
The international community has pressed for a vote to choose a new head of state to be held as soon as possible -- but no timetable has been set so far.
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