Political tensions mounted across Nigeria on Thursday with the announcement by the Independent National Electoral Commission (INEC) that the 24 candidates cleared to run in April’s presidential election do not include Vice President Atiku Abubakar, who fell out with outgoing President Olusegun Obasanjo and is currently being indicted on charges of corruption.

“Once you have been indicted, INEC cannot do anything about it,” the chairman of the electoral body, Maurice Iwu, told reporters, citing Nigeria's 1999 constitution.

Iwu denied allegations that he deliberately froze out Abubakar and said that INEC would only authorise Abubakar’s candidacy under court order.

Abubakar supporters have threatened that a disqualification would lead to election chaos and observers have warned of potential violence surrounding the presidential polls in many parts in the country.

Since announcing he would run in December Vice President Abubakar had been considered one of the leading candidates, rallying massive crowds in cities around the country.

He broke away from the ruling People's Democratic Party (PDP) to join the Action Congress party in December, taking with him some of the PDP’s highest ranking members, including two former PDP chairmen.

Abubakar fell out with Obasanjo last year after he opposed plans to amend the constitution to eliminate the current two-term presidential limit. Abubakar was subsequently indicted by Nigeria's Economic and Financial Crimes Commission for allegedly diverting more than US$125 million of government funds to his personal businesses - charges Abubakar strongly denies.

The candidates cleared to run in the election include Umaru Yar'Adua, the flag-bearer of the PDP and current governor of northern Katsina State; Muhammadu Buhari, a former military ruler and candidate of the main opposition All Nigeria People's Party; and Chukwuemeka Ojukwu, who led the failed secession of southeast Nigeria as Biafra in the 1967-70 civil war, is the candidate for the All Progressive Grand Alliance.

A total of 50 political parties have registered for the elections but not all fielded presidential candidates.

If the elections are successful, it would be the first time since Nigeria became independent from Britain in 1960 that power would transfer from one elected government to another.

Mismanaged elections in 1965 led to the country's first military coup a year later and nearly three years of civil war in which more than one million people died, most of them from starvation. Another disputed election in 1983 triggered a military coup, leading to 15 years of military rule, which ended in 1999 with the election of Obasanjo, a former military ruler.