Nigeria's stock market, Africa's best performer during the past decade, posted the biggest declines worldwide in the first quarter as bad loans to speculators pushed bank valuations to an all-time low.

The Nigerian Stock Exchange All Share Index fell 37 percent this year, the steepest quarterly decline in more than a decade and the worst of 89 benchmark indexes tracked by Bloomberg. Stocks in Africa's largest oil-producing nation reached a five-year low last week, even as a rebound in crude spurred gains in commodity-exporting countries from Russia and Norway to Brazil.

Investors have been fleeing "the good, the bad and the ugly" of the financial industry since Nigerian regulators allowed banks to delay booking losses on so-called margin loans backed by shares, emerging-markets brokerage Renaissance Capital says. The lack of disclosure left investors unable to identify potential losses. The All Share Index may fall another 9 percent, according to Moscow-based Renaissance and London-based Exotix Holding Ltd.