UMNO Youth chief Ahmad Zahid Hamidi said he sent a letter of resignation to Prime Minister Mohamad Mahathir on Tuesday, a day before his release.

Zahid said he sent a second letter confirming his decision to Mahathir on Thursday. Mahathir is president of the United Malays National Organisation (UMNO), Anwar used to be deputy president.

``The decision to resign is not due to any particular party or UMNO leader,'' he told a news conference. ``It is due to the realisation of the importance of party unity and the sacrifice I had to make as leader of the youth wing.''

``As a leader with responsibility, I thought resigning was something I had to do,'' he said. ``I will continue to work for the party as a member and I will remain in UMNO. I hope my decision is respected by all parties.''

Zahid wasarrested in Kota Kinabalu in the eastern state of Sabah on September 21, a day after police stormed Anwar's home in Kuala Lumpur and detained him following a massive anti-Mahathir rally in the capital.

Anwar was dismissed as deputy prime minister and finance minister on September 2 and was kicked out of the ruling party two days later. He appeared in court this week charged with 10 counts of sodomy and corruption. He denied all charges.

Zahid was among five people released on Wednesday. Others were from the Islamic Youth League. All were detained under the draconian Internal Security Act (ISA), under which Anwar is also being held.

The act, strongly criticised by human rights groups, provides for indefinite detention without trial.