CONAKRY (Reuters) - Guinea's long-delayed legislative election aimed at completing the mineral-rich West African nation's transition to democracy will be held on September 24, its electoral commission said on Tuesday.

Guinea's government and opposition parties reached a U.N.-mediated agreement last week to hold elections at the end of September following a wave of opposition protests accusing President Alpha Conde of planning to rig the polls.

"The Independent Electoral Commission has proposed holding the election on September 24," said a statement from the body read over Guinea's state television late on Tuesday.

Under Guinean law, President Conde, who was elected in 2010 in Guinea's first democratic election since independence from France in 1958, must now accept or reject the proposed date.

The elections, initially planned for 2011, have been postponed several times as a result of disputes between the opposition and the government over preparations for the vote.

Guinea's opposition leaders have staged a series of protests in the capital Conakry since March, in which more than 50 people have been killed and 350 wounded.

Political instability following a 2008 military coup has deterred some investors from putting money into Guinea despite its large deposits of iron ore, bauxite, gold and other minerals.

(Reporting by Saliou Samb; Writing by Bate Felix; Editing by David Brunnstrom)