There were riots in Lahore today as as the head of the Pakistan Muslim League party (PML-N) Nawaz Sharif, claiming he was under house arrest, left his home to take part in a march from Lahore to Islamabad to demand the reinstatement of judges who were fired by former President Musharraf.

Meanwhile, the government of Asif Ali Zardari, husband of the late Prime Minister Benazir Bhutto, denies that Sharif is under house arrest and has moved to block the roads to prevent his supporters from reaching Islamabad.

Ahead of the protest, the government has also arrested hundreds of opposition activists and banned rallies, saying they could trigger violence.

Our Islamabad correspondent says the campaign over the judges has become a power struggle between Mr Sharif and current President Asif Ali Zardari.

She says the unrest has alarmed the West, which wants Pakistan to focus on the battle against the Taleban on the Afghan border.

President Zardari - the widower of former Prime Minister Benazir Bhutto - promised to bring back the judges when he took office last year following his wife's assassination.

Early on Sunday, riot police blocked access roads to Mr Sharif's home and reportedly baton-charging his supporters.

Leaving his home later in the morning, Mr Sharif told a crowd: "The house arrest is illegal and immoral. All these decisions are unconstitutional," reported AFP news agency.

The government says it stationed troops outside Sharif's home "for his protection" as a result of terrorist threats and that he was not under "house arrest."

Sharif, once a member of Zardari's governing coalition but has since abandoned his ally over the judges issue, was recently the subject of a supreme court ruling that barred he and his brother from seeking elected office. A notoriously corrupt politician, it was Sharif who Musharraf overthrew in 1999 to set up his military government.

Needless to say, with the Northwest Frontier Province in turmoil, Pakistan does not need this internal squabbling.