LONDON (AFP) - Lawyers for Pakistan's former president Pervez Musharraf said Friday a treason charge levied against him was politically motivated and that he would face a "show trial", urging the United Nations to intervene.

The legal team also called on the United States, Britain and Saudi Arabia to denounce Musharraf's trial to "repay their debt" for his support in the US-led "war on terror" in the wake of the 9/11 attacks.

Musharraf overthrew the government of current Prime Minister Nawaz Sharif in a bloodless military coup in 1999 and ruled until 2008. His lawyers claim Sharif is using the treason case to get revenge.

The 70-year-old ex-commando, who is due to appear before a special court on December 24, is the first former military dictator in Pakistan's history to face trial for treason.

Musharraf's barrister Steven Kay told a press conference in London the hearing would be a "stage-managed show trial" with the judges picked by political opponents who are now in power.

"What we have here is a case that has started with the hand-picking of judges by the politicians - or a politician, the prime minister - in defiance of any person's right to a trial that is fair," Mr Kay told reporters.

The trial is an "egregious example of political interference," he said.

Musharraf returned to Pakistan from self-imposed exile in March and was placed under house arrest.

The treason accusation relates to his decision in 2007 to impose emergency rule shortly before the Supreme Court was due to decide on the legality of his re-election as president a month earlier, while he was still army chief.

Mr Kay said the judges selected for the trial would be unable to act impartially - particularly since one of them, Faisal Arab, was sacked by Musharraf's government.

"If you've been affected by what took place and then you judge it, there is a conflict of interest because you have an interest in getting retribution," he told reporters.

The legal team has written to UN human rights chief Navi Pillay and UN special rapporteurs calling for the international body to "urgently intervene and ensure that the former president is not subjected to politically motivated charges".

Barrister Toby Cadman called on Musharraf's allies in the "war on terror" to support him, insisting this did not amount to interference in Pakistani justice.

"What is important is to ensure that voices of concern are expressed in terms of the charges which the former president faces, and recognition of his contribution to the 'war on terror'," Mr Cadman said.