Nigeria, which ended military rule in 1999, has suffered from a series of military coups since winning independence from Britain, including one led by Buhari in 1983.
While in office, Buhari, a 75-year-old former general, has worked to assure Nigerians he is committed to democracy.
"I tender the nation's apologies to Chief MKO Abiola," for the cancellation of the June 12, 1993 election, Buhari said during an event in honor of the politician who died in July 1998.
While observing a minute's silence in memory of Abiola and other pro-democracy heros who died during the struggle, Buhari said "Nigeria will no longer tolerate such perversion of justice."
The government said that from next year, June 12 would be a public holiday and commemorated as "Democracy Day" instead of May 29, the date marking Nigeria's return to civil rule in May 1999.
He died mysteriously on July 7, 1998 as talks got under way for his release.
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