Unidentified gunmen have assassinated a senior leader of the main Nigerian opposition party, raising fears of more violence ahead of April's presidential election.

Police and party officials say the gunmen shot and killed a senior official of the All Nigeria People's Party, Harry Marshall, at his home in Abuja Wednesday.

Mr. Marshall had been a leading figure in the presidential campaign of Muhammadu Buhari, the main challenger to President Olusegun Obasanjo.

Police say the identity of the gunmen was not immediately known, and an investigation has been ordered. Spokesmen for the All Nigeria People's Party are blaming President Obasanjo's People's Democratic Party for the attack.

Mr. Marshall had been a founding member of the ruling party, but he quit last year in a dispute with Governor Peter Odili of his native Rivers state, a key southern oil producing region.

Mr. Marshall had been coordinating Mr. Buhari's plans to launch his campaign with a rally in Port Harcourt in Rivers state on Saturday.

The opposition candidate, Mr. Buhari, is a former military ruler of Nigeria. He comes from the Muslim-dominated north and supports Sharia law, the Islamic criminal code. He is considered the strongest contender to incumbent President Obasanjo, an evangelical Christian from the southwest who is seeking a second term in the April 19 election.

The Marshall assassination raises fears of more violence as Nigeria tries to hold its first election under civilian administration in more than 20 years. More than 10,000 people have been killed for religious, ethnic or political motives since President Obasanjo took office in 1999.