Former Malaysian leader Mahathir Mohamad has quit the ruling party, saying it has been hijacked by embattled Prime Minister Najib Razak to protect his own interests.

Najib has come under pressure to resign after documents leaked last July showed more than $700 million channeled into his private accounts. The attorney general decided last month not to prosecute Najib, saying most of the money was a personal donation from Saudi Arabia's royal family.

Mahathir said Monday that he quit the United Malays National Organization because it was now "a party dedicated to supporting Najib, protecting Najib and upholding whatever he does, including some of the wrong things he has done."

Mahathir, who ruled for 22 years before stepping down in 2003, has led calls for Najib to resign.