ABUJA, May 26 (AFP) - Retired general Olusegun Obasanjo will be sworn in on Saturday as the first elected president of Nigeria in more than 15 years. Situated on the coast of west Africa, Nigeria is home to around 250 different ethnic and linguistic groups and is the world's most populous black nation. GEOGRAPHY: Four times the size of Britain, Nigeria lies in west Africa, bounded by the Atlantic Ocean and the republics of Benin, Niger, Chad and Cameroon. Mainly low-lying, the north is largely semi-desert giving way to a tropical forest belt in the southwest and a vast riverine region in the southeast. POPULATION: The population is estimated at 108 million by the Nigerian government and 121.8 million by the United Nations. It's three largest ethnic groups are the Hausa-Fulani in the north, Ibo in the east and Yoruba in the southwest. CAPITAL: Abuja. Major towns include the commercial hub Lagos, Enugu, Ibadan, Kaduna, Kano, Port Harcourt and Sokoto. GOVERNMENT: A presidential system, backed by a two-chamber parliament, with separate assemblies for the 36 states of the federation. LANGUAGE: The official language is English but most people speak several other languages, with the most widespread being Hausa, Ibo and Yoruba. RELIGION: The main religions are Moslem and Christian with some animist religions continuing to flourish. HISTORY: Formed in 1914 by Britain, Nigeria gained independence in October 1960 and declared itself a federal republic three years later. After two successive military coups in 1966, the country plunged into a civil war in 1967. The war lasted for almost three years during which more than a million people died. A civilian regime was put in place in 1979 but was overthrown in 1983. Nigeria has since been run by a series of military regimes. ECONOMY: In free-fall. The economy is based on oil exports with other sectors long neglected. Most of the population is involved in subsistence farming. CURRENCY: The naira, currently traded at an official rate of 94.88 to the dollar. -=-=- 