* Debtors question auditors' conclusions
* Legal wrangling could delay clean-up of banking sector
* SEC seeks answers from stock exchange boss
(Recasts with statements from Dangote, Oando)
By Nick Tattersall and Oludare Mayowa
LAGOS, Aug 20 (Reuters) - Challenges mounted on Thursday to efforts by Nigeria's central bank to recover almost $5 billion of bad debt, with some companies and directors denying liability and one banking executive seeking legal redress.
The central bank (CBN) on Wednesday published a list of more than 200 firms, individuals and state bodies which it said were defaulting debtors of five banks it bailed out on Friday with a $2.6 billion capital injection.
The list itemised non-performing loans totalling 747 billion naira ($4.97 billion) with Afribank, Finbank, Intercontinental Bank, Oceanic Bank and Union Bank.
Anti-corruption police have given debtors a week to organise repayment or face arrest, prosecution and asset seizures.
The names listed as directors and shareholders in some of the defaulting companies read like a Who's Who of Nigeria's corporate aristocracy, including the only two Nigerians on the latest Forbes billionaires list, Aliko Dangote and Femi Otedola.
Analysts have said the move by new central bank governor Lamido Sanusi, who took office just two months ago, could be the start of a long-overdue clean-up of corporate governance and banking regulation in sub-Saharan Africa's number two economy.
But his efforts could be hamstrung by drawn-out wrangling among powerful corporate and political interests, particularly if bankers or tycoons decide to challenge his auditors' assertions in Nigeria's sluggish courts.
"It increases confidence in corporate disclosure, which is critical to the long-term success of both the equity and corporate bond markets ... (but) there remain risks," Mike Hugman, London-based emerging markets strategist at Standard Bank, said of the central bank's move.
"There will certainly be political challenges to improved corporate governance."
Dangote Industries issued a public denial on Thursday that its founder, one of Nigeria's most powerful business figures and estimated by Forbes to be worth $2.5 billion, was a director or shareholder of Dansa Oil and Gas Limited, listed by the CBN as a defaulting customer of Intercontinental Bank.
It also said it was in dispute over charges relating to its own 2.5 billion naira debt to Oceanic Bank, listed by the CBN as non-performing, but the matter was very close to resolution.
"It is on record that our credit rating remains admirable and our bankers have confidence in our ability to meet our obligations," it said in a statement.
CONCLUSIONS CHALLENGED
Nigerian energy firm Oando denied it had a 7.1 billion naira non-performing loan with Oceanic Bank and called on the central bank and Oceanic to correct the "erroneous impression" created by its inclusion on the list.
It published a table detailing its balances at the end of May, when the central bank audit was completed, and by Aug 14.
This Day newspaper said Intercontinental's new management had declared a 16 billion naira loan to United Alliance Company performing, contrary to the central bank's assessment.
Aigboje Aig-Imoukhuede, the head of Access Bank, was listed by the CBN as the director of United Alliance.
The central bank has not responded to specific debtors' claims but said its list was based on balances at the end of May.
"If any defaulters/debtors have made repayments after that date, they should sort it out with the relevant bank," it said.
One of the biggest debtors to Union Bank and Intercontinental Bank was conglomerate Transcorp, of which Ndi Okereke-Onyiuke -- the director-general of the stock exchange -- is the chairman.
The Securities and Exchange Commission (SEC) on Thursday gave Okereke-Onyiuke seven days to explain herself. It did not specify what sanctions it might take but has the power to suspend her from the stock exchange.
The Economic and Financial Crimes Commission (EFCC) has also been questioning the top management of the rescued banks.
Analysts say criminal charges could be brought if executives are found to have falsified accounts or breached share price manipulation rules by setting up subsidiaries as vehicles to trade their own stock and push the share price higher.
Local media have reported that one of the sacked chief executives, former Intercontinental boss Erastus Akingbola, has won a reprieve from a federal high court allowing him to challenge his removal by the central bank.
Akingbola could not be reached for comment. (For more Reuters Africa coverage and to have your say on the top issues, visit: http://af.reuters.com/ ) (Writing by Nick Tattersall; Editing by Lin Noueihed)