LAGOS (AFP) -- Nigeria, Africa's most populous nation, is about to embark on a controversial census which some fear could reignite the ethnic, regional and religious tensions which threaten to tear it apart.

President Olusegun Obasanjo is determined to find out just how many Nigerians there are -- estimates range from between 120 and 150 million -- despite fierce opposition from various powerful vested interests.

But preparations for the March 21 to 25 exercise, the first such headcount since 1991, have already laid bare many of the deep rifts in Nigerian society that have fuelled a seven-year-old wave of unrest that has left 20,000 dead.

Last week, Information Minister Frank Nweke warned that unidentified groups plan to incite "civil disturbances in some states with a view to disrupting this month's national census" and warned of a police crackdown.

That Nigeria is the giant of Africa is not in doubt. This highly urbanized, oil-rich west African melting pot is home to almost twice as many people as its nearest rivals on the continent: Ethiopia and Egypt.

The United Nations Population Fund put Nigeria's population in 2005 at 131.5 million. The Central Intelligence Agency reports 128,771,988.

And, when Obasanjo was lobbying the foreign governments for debt relief last year, he said he was doing so in the name of 150 million people.

Nigerians, however, are not so much interested in how many countrymen and women they have, but rather in who exactly they are.

There is a constant struggle for influence between three main ethnic groups -- the Yoruba, Hausa and Igbo -- while the roughly 50-50 split between Muslims and Christians stokes further tensions.

Many groups want to maximize their turnout in the census to push their case for a greater share in government funding and political influence. Others do not want to be counted at all.

"The Igbos are not part of the census because we are not Nigerians. We are Biafrans," Okechukwu Nwaogu, a spokesman for the banned Movement for the Actualisation of the Sovereign State of Biafra (MASSOB), told AFP on Monday.

In 1967 the Igbos of southeastern Nigeria attempted to split from Nigeria and form a breakaway Republic of Biafra. They fought and lost a brutal three year revolt against federal forces, which left one million dead.

Last month, during a sectarian riot in the southern city of Onitsha, machete-wielding Christian Igbo youths once more chanted "Biafra, Biafra!" as they burned the bodies of their slaughtered Muslim Hausa neighbors.

Lagos -- which probably edges out Cairo as Africa's biggest city -- is therefore keen that its cosmopolitan population stays in town to register rather than dispersing to various traditional homelands.

Authorities have imposed a three-day travel ban and threatened to punish anyone who tries to slip out of the city to be counted in their ancestral home.

The oft-delayed census comes at a time of high political tension.

A committee of lawmakers has proposed a constitutional reform to allow a president to stand for a hitherto banned third term -- a move widely seen as a ploy by Obasanjo to allow himself to stay in office until 2011. Last month, U.S. Director of National Intelligence John Negroponte warned that Obasanjo's alleged plan "threatens to unleash major turmoil and conflict."

"Such chaos in Nigeria could lead to disruption of oil supply, secessionist moves by regional governments, major refugee flows, and instability elsewhere in west Africa," he told senators.

Many in Nigeria associate the census with Obasanjo's alleged maneuverings.

"There is every reason to believe that the census is an exercise to aid Obasanjo's agenda. Otherwise, why is it coming up at the tail-end of his second term? We smell a rat," said Lagos rights activist Sina Loremikan.