KUALA LUMPUR, Malaysia (AP) - Malaysia's ousted deputy premier used an opposition party platform Wednesday for his most critical attack on the government, suggesting Prime Minister Mahathir Mohamad feared declining popularity and could be investigated for corruption.

As about 1,500 ethnic Chinese chanted, ''Reformasi,'' the Malysian word for ''reform,'' Anwar Ibrahim denied allegations against him of treason, abuse of power and sexual misconduct, saying his only sin was ''being a viable alternative to Dr. Mahathir.''

Shouting through a microphone, surrounded by former opponents from the Democratic Action Party, Anwar made one of his boldest speeches since Sept 2, when Mahathir fired him as his deputy and finance minister, then engineered his ouster from the ruling party.

''If you want to accuse me of corruption, then you should first investigate the other ministers, starting with Dr. Mahathir,'' Anwar said, issuing a challenge to the authoritarian prime minister. ''You have chosen the wrong guy to pick a fight with.''

''Mahathir is scared because his popularity has declined,'' Anwar said.

Ignoring threats by the national police chief to arrest more of his associates and press charges against him if he continues holding rallies without police permission, Anwar planned to continue his nationwide tour on Thursday, taking his campaign for reform to the east coast.

His appearance on an outdoor platform with opposition leader Lim Kit Siang was legal because the Democratic Action Party has obtained permission for nightly vigils demanding the release of Lim's son.

Lim Guan Eng recently began serving an 18-month prison sentence for sedition and spreading false news because he wrote and distributed a pamphlet criticizing the judiciary and the attorney-general.

Anwar, expecting to soon be facing that judicial system himself, said of Lim Guan Eng, ''You don't finish off a young, qualified man just because he tends to be more critical and he intends to expose your wrongdoings.''

While denying that he had joined forces with the opposition, Anwar said they agree on promoting justice and the need for reform.

Anwar signed a petition demanding Lim Guan Eng's release and shook hands with some of the dozen protesters conducting a rotating fast at the opposition party headquarters to press for the young man's freedom.

''In Malaysia the opposition and the government have never mixed. This is a historic moment,'' said Steven Chan, one of those fasting.

Earlier, the inspector-general of police, Abdul Rahim Noor, said charges would be pressed against Anwar if he continued to hold his rallies without the proper police permits.