Police in Congo have opened fire on protesters demonstrating against the president's bid for a third term in office.

The clashes in Brazzaville on Tuesday erupted after authorities banned a planned protest rally against a referendum Sunday on President Denis Sassou Nguesso's quest to extend his three-decade stay in office.

Earlier, communications were cut in Congo's capital Brazzaville, just hours ahead of the protest.

Shops remained shuttered and schools and offices were closed across most of the city as young protesters took to the streets and torched tyres in protest in Brazzaville's southern district of Makelekele and western district of Bacongo.

Police opened fire several times with the Reuters news agency reporting four people had died.

The Associated Press news agency did not report deaths but said seven people had been injuured by teargas and bullets.

Sources at Makelekele hospital said seven people were treated for injuries, five with serious gunshot wounds and two hurt by teargas grenades. A correspondent from the AFP news agency saw another person being brought to the hospital after  being shot in the abdomen.

Amnesty International condemned the use of force and demanded authorities refrain form using excessive force against protesters.

"A heavy-handed response by security forces not only violates the protesters' rights to freedom of expression and peaceful assembly, but may enflame an already tense situation," said Ilaria Allegrozzi, Amnesty International Central Africa researcher.

The protest was being held ahead of Sunday's referendum on a constitutional reform that will determine whether veteran leader Denis Sassou Nguesso - who has already had three decades in power - can seek another presidential term.

The constitution currently bars the 72-year-old from running again as there is an age limit of 70 as well as a maximum of two terms.

Tens of thousands of the president's supporters staged a rally on Saturday to back the constitutional changes.

Source: Agencies