Thousands of Pakistani opposition activists defied a ban on protests and launched a sit-in outside parliament on Thursday.

People flocked to Islamabad from cities across Pakistan, despite attempts by riot police to prevent them from taking part in the mass demonstration, which is calling for the reinstatement of judges sacked by former president Pervez Musharraf in 2007.

In Karachi, police baton-charged protesters and arrested around 60 outside the high court, where they had been planning to leave for Islamabad.

Information Minister Sherry Rehman said that the government had decided to ban opposition protests in order to ‘avoid bloodshed in the streets.’

While acknowledging that her Pakistan Peoples Party (PPP) had staged similar rallies in the past, she insisted that ‘we never called to raise the flag of rebellion.’

The ruling PPP has restored most of the judges fired by Mr Musharraf, but a few, including a former Supreme Court chief justice, have not regained their seats.

Pakistan President Asif Ali Zardari, who has turned out to be a staunch US ally, is believed to fear that those judges could move to limit his power or reopen corruption cases against him that were dropped by Mr Musharraf when the former general was seeking to forge political alliances before last year’s elections.

With the showdown threatening to destabilise the country’s one-year-old civilian government at a time of rising militant violence in the country’s volatile tribal areas, US ambassador Anne Patterson met the leader of the opposition Pakistan Muslim League-N party Nawaz Sharif in a bid to resolve the crisis.

Mr Sharif is furious over a court decision last month that barred him and his brother from office.

After the ruling, the federal government dismissed the Punjab provincial administration led by Mr Sharif’s brother, stoking popular anger at Mr Zardari.

The growing political unrest is raising the spectre of a possible military intervention in the nuclear-armed country.

It could put Washington in an awkward position if the civilian government – which itself rose to power on the back of the rallies and marches against Mr Musharraf – keeps cracking down on dissidents.

Mr Zardari, the widower of assassinated former prime minister Benazir Bhutto, has cultivated close ties with Washington and has sought to rally Pakistanis behind the fight against Islamist guerillas.

Mr Sharif is considered to be closer to Islamist parties and political factions opposed to the US-backed campaign.