KUALA LUMPUR, Sept 15 (Reuters) - Malaysia's sacked finance minister Anwar Ibrahim said on Tuesday that authorities had stepped up pressure on him by detaining his former private secretary, and he expected to be arrested soon for treason.

Anwar said Mohamed Ahmad, his former private secretary in the Finance Ministry, was arrested before dawn on Tuesday at his home outside of Kuala Lumpur and was being held without charges.

Anwar, who was sacked as deputy prime minister and finance minister on September 2, said a former speechwriter had been detained on Monday, eight days after his adopted brother Sukma Darmawan was arrested and held without charges.

Last month, a businessman friend of Anwar's was arrested under the Internal Security Act for unlawful possession of ammunition and firearms. He has been kept in detention since his arrest.

The former cabinet member, under investigation over allegations of sexual misconduct and treason, said he believed the police moves were part of an effort to incriminate him and foreshadowed his own arrest without bail.

''Since there is no clear security case here, there is one explanation -- to incriminate me,'' Anwar told a news conference in his suburban home outside of Kuala Lumpur.

He predicted he would be arrested after the 16th Commonwealth Games, which end on September 21, and a visit next week by Britain's Queen Elizabeth.

''They will wait until Her Majesty the Queen leaves, and then they will probably do what is necessary,'' said Anwar.

Anwar, once heir apparent to Prime Minister Mahathir Mohamad, said authorities were preparing 21 charges, including that of treason, against him and that he expected to be indicted on non-bailable offences.

''If they were bailable charges, I would continue to speak up,'' said Anwar, who has repeatedly said he was framed by opponents worried about his political ascendancy and opposed to his calls for an end to cronyism and corruption.

Mahathir has said he sacked Anwar because he was morally unfit but the prime minister has given no specific reasons. One day after Anwar was dismissed from the cabinet, Mahathir's United Malays National Organisation (UMNO) expelled him.

Anwar said his adopted brother and former aides had been arrested to incriminate him.

''The public cannot be duped into believing that this is either a security case or a case of sexual misconduct or treason. This is purely a political case, to assassinate me politically and to deny me of my rightful place in the party,'' he said.

''It is a conspiracy of the highest level in the government.''

Anwar pledged to continue speaking to supporters despite a warning by police on Monday that the gatherings were disturbing his suburban neighbourhood. Anwar said he planned to deliver a speech on Tuesday evening to sympathisers south of the capital.

''I have made up my mind. I will not refrain from speaking.''

Anwar travelled to three states over the weekend to address thousands of sympathisers, who have taken up the slogan "Reformasi'' (Reform).

Although authorities have not given him a permit, usually required for political gatherings of more than three people, they have not intervened. But on Monday evening a senior police officer approached him about the noise during the meetings at Anwar's home.

''This idea of a police permit, although assured by the prime minister, is a political gimmick,'' said Anwar, flanked by his wife and in bare feet.

Anwar said the family of his former private secretary had been told by police not to inform the sacked cabinet minister of the arrest, and his lawyers would produce a writ of habeus corpus to have Mohamed produced in court.