ISLAMABAD (AFP) - Pakistan's former military ruler Pervez Musharraf was rushed to hospital on Thursday after suffering a "heart problem" on his way to court for a hearing in his treason case.

The 70-year-old had been summoned to the special tribunal in Islamabad after failing to show up for two previous sessions due to security threats against him.

Musharraf's team says the allegations, which relate to his imposition of emergency rule in November 2007, are politically motivated and his lawyers have challenged the authority of the three-judge tribunal.

Jan Mohammad, a senior police official, told the court in Islamabad that Musharraf had fallen ill while being transported to the hearing under heavy security.

"He has been shifted to Armed Forces Institute of Cardiology after he got heart problem," Mohammad said.

An aide to Musharraf, who is facing a series of criminal cases dating back to his 1999-2008 rule, told AFP the retired general was in "bad shape".

Earlier on Thursday his lawyers walked out of court, complaining of being threatened and harassed.

Anwar Mansoor Khan, one of the lawyers, told the court he has been receiving threats and was unable to sleep the night before the hearing.

"I was under total threat... from 1:00 am to five in the morning. Someone was banging on my door and ringing my bell," Khan told the court.

When one of the judges asked who was threatening him, Khan answered: "This very government." The court promised to investigate but Khan walked out of court, followed by other members of Musharraf's legal team.

"This never happened in my 40 years of practice. I will walk out," Khan said.

Musharraf's lawyers have previously said the treason case is an attempt by the government of Prime Minister Nawaz Sharif, whom Musharraf ousted in a coup in 1999, to settle old scores through the courts.

Sharifuddin Pirzada, another of Musharraf's lawyers, also complained that he had been threatened.

Khan told the court on Wednesday he had been attacked in his car while travelling to the eastern city of Lahore following an earlier hearing.

The treason allegations are the latest in a series of criminal cases faced by Musharraf since he returned to Pakistan in a thwarted bid to run in last May's general election.

These include murder charges over the assassination in late 2007 of former prime minister Benazir Bhutto.

Musharraf is the first former army chief to go on trial in Pakistan, setting up a potentially destabilising clash between the government - which brought the charges - and the all-powerful military.

On Sunday the retired general denounced the treason case as a "vendetta" against him and claimed he had the backing of the military.