A leading figure in Japan's governing party, Ichiro Ozawa, says he will not resign despite a growing scandal over political funding.
Police have now arrested three aides to Mr Ozawa, the secretary-general of the Democratic Party, seen by many as Japan's most influential politician.
Mr Ozawa admits misunderstandings over a suspicious land deal but insists he has done nothing wrong.
The DPJ came to power in September but has been dogged by corruption claims.
The BBC reports Mr Ozawa is credited with engineering last year's election victory which ended half a century of conservative dominance. Since then, the government has leaned heavily on his skills.
The accusations surround Mr Ozawa's political fundraising organisation and a purchase of a plot of land in the capital Tokyo.
Prosecutors are investigating the origin of about 400 million yen ($US4 million) used to buy the land in 2004.
Prime Minister Yukio Hatoyama has said he is standing by Mr Ozawa.