He said the company had uncovered years of financial misconduct including under-reporting of income and inappropriate personal use of company assets.
Renault said its board would meet "shortly", after Ghosn was detained over allegations including underreporting his income.
"Ghosn is greedy", a Nissan executive officer said.
At a press conference on Monday, the Nissan CEO said he felt "despair, indignation, and resentment". Neither automaker has publicly spoken about the future of his roles. Management, including Saikawa, was strongly alarmed by moves for the two companies to completely integrate their management in keeping with the French government's intentions, and nervously watched every step Ghosn made.
He also is suspected of underpaying his share of the rent and misusing company funds on his company-issued housing in Lebanon, Paris, Amsterdam and Rio de Janeiro, where his apartment is on the wealthy Copa Cabana beach strip. Nissan's German-listed securities plunged 12 per cent. The country owns around 15 percent of Renault, Nissan's partner, and supported Ghosn's renewal at the helm of the French automaker.
The departure of the alliance's architect, who has been at the helm since 1999, would have been destabilizing in the best of circumstances.
In recent years, however, there has been discord in the company with regard to Ghosn.
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He is credited with transforming the company from near-bankruptcy to profitability within two years and steering the vehicle maker into an expansion in China, one of Nissan's key markets by volume.
The news of Ghosn's downfall emerged unexpectedly on Monday evening, with local media first reporting he was being questioned by prosecutors and that Nissan's headquarters was being raided.
The scandal was a "negative outcome of the long regime of Mr. Ghosn", he said.
If the misconduct continued for years, as Saikawa said, analysts question how this could have passed undetected for so long, unless internal auditors and senior management turned a blind eye.
Ghosn was one of the first executives to champion electric cars, and he also developed a range of low-priced cars for emerging markets.
In June, Renault shareholders approved Mr Ghosn's €7.4m pay for 2017.
The compensation from Nissan was only a part of Mr Ghosn's overall income since he also was paid salaries by the auto maker's alliance partners Renault SA and Mitsubishi Motor Corp.
Monday's news marks a stunning downfall for an executive long-regarded as the man who saved Nissan almost 20 years ago. That opens up a leadership void at the entire alliance, for which Ghosn officially still serves as CEO and chairman.