Security forces in Congo Republic's capital fired warning shots and tear gas to disperse hundreds of protesters marching against a referendum they say is a ploy to extend the president's grip on power, a day after police killed four opposition supporters.

President Denis Sassou Nguesso's critics accuse him of seeking to use a referendum on Sunday on whether to lift a limit on the number of terms a president can serve to extend his rule ahead of an election next year.

Four people were killed on Tuesday when police opened fire on a crowd of protesters.

On Wednesday, the crowd marched towards the police station in the southern district of Makelekele chanting anti-government slogans and "We will rise up" in the local Lari language.

Residents carrying their belongings on their heads fled the area amid a thick cloud of tear gas, a Reuters witness said.

Interior minister Raymond Zephyrin Mboulou said on state radio late on Tuesday that 16 people were arrested in connection with Tuesday's protests. Three security forces were seriously injured and the houses of five politicians set ablaze, he said.

Burundi has suffered months of violent turmoil since President Pierre Nkurunziza announced in April he would seek a third term, a move that triggered protests and a failed coup.