He said he would ask the authorities to allow him to visit Suu Kyi, and thanked the UN, EU and others for pressing for his release.

The deputy chief had spent nearly seven years in prison and under house arrest. The government renewed his detention on an annual basis following his arrest in 2003.

UN plea

Ban Ki-moon, the UN secretary-general welcomed Tin Oo's release and said he hoped it would promote "substantive dialogue" between the NLD and the government.

He also called for the lifting of restrictions on Suu Kyi "without further delay" as well as the release of other political prisoners.

Human rights groups say the government still holds some 2,100 political prisoners.

"The fact that there still [so many] political prisoners in Myanmar and if the elections are to be credible, they need to be able to participate along with all representative groups in the elections," Andrew Heynes, the British ambassador, said after Tin Oo's release.

"The release of Tin Oo is very welcome, but we should not attach any political significance to the release," Mark Farmaner, director of the rights group Burma [Myanmar] Campaign UK, said.

"Burmese democracy activists are regularly released when the generals want to score points with the international community, and are then arrested again later," he said.

Tomas Ojea Quintana, the UN special envoy to Myanmar, is due to arrive on Monday.

'Unfair constitution'

Suu Kyi has been detained for 14 of the past 20 years. Her house arrest was extended in August by an additional 18 months, which would prevent her from taking part in the first national elections in 20 years. The government says the vote will be held in 2010.

Suu Kyi's party won the last elections in 1990 by a landslide, but the military, which has ruled Myanmar since 1962, refused to give up power and has constantly obstructed her party's operations over the past two decades.

Suu Kyi's party has not yet decided whether to take part in the upcoming elections. The party says the new constitution of 2008 is unfair and will perpetuate military rule, a claim backed by international rights groups.

The constitution guarantees that 25 per cent of parliamentary seats will go to the military. It also has a clause that would effectively bar Suu Kyi from holding office.