The campaign for next month's presidential election in Republic of Congo begins on Saturday, with 82-year-old President Denis Sassou Nguesso the clear favourite.

The career military officer first led Republic of Congo, which is also known as Congo-Brazzaville, under the one-party system from 1979 to 1992, before losing the country's first multi-party elections to former prime minister Pascal Lissouba.

He overthrew Lissouba in a civil war to return to power in 1997.

Earlier this month, he announced he would be seeking a new five-year term, which, according to the constitution, would be his last.

Campaigning begins on Saturday on the Atlantic coast in the country's economic and oil capital, Pointe-Noire, with a rally held by the president's Congolese Labour Party (PCT).

It will end on March 13, with the first round of voting on March 15.

Six opposition candidates have formally confirmed they will be standing, including 34-year-old Destin Gavin, from the Republican Movement (MR), who is in the race for the first time.

But the fragmented opposition stands little chance of winning, with the ruling party promising a "wave" in favour of its "patriarch", according to roadside campaign posters.

Sassou Nguesso is one of Africa's longest-ruling leaders after Paul Biya of Cameroon, who has been in office since 1982, and Equatorial Guinea's Teodoro Obiang Nguema Mbasogo, who took power in a 1979 coup.