LAHORE: Pakistan's main opposition leader Nawaz Sharif defied house arrest on Sunday, ramping up his challenge to the government, as riot police clashed with stone-throwing mobs in street battles.

The former premier, now the most popular political leader in the country, led about 10,000 supporters in a banned protest in Lahore, from where he has urged thousands of activists to march on the capital Islamabad by Monday.

Sharif, locked in a standoff with President Asif Ali Zardari since the Supreme Court on February 25 barred him from running for office, is demanding the government reinstate judges deposed by ex-military ruler Pervez Musharraf.

‘We don't accept this decision. The house arrest is illegal and immoral. All these decisions are unconstitutional,’ he told a crowd after breaking his house arrest order.

‘Come and join me. I am leaving the house. The time has come to march hand in hand.’

In the most violent scenes since the crisis began, riot police wearing body armour baton-charged protesters and fired tear gas. Witnesses said more than a dozen people were wounded.

‘The main GPO Square looked like a battleground. I saw at least two ambulances ferrying casualties to the hospital,’ said resident Hanif Goraya, as Sharif supporters brought the city centre to a standstill.

‘Police fired scores of shells, inside and outside the Lahore High Court building. A shell hit my left thigh, I received stitches. The injured include lawyers, political workers and some police officials,’ he said.

At one point, crowds surged ahead of Sharif's convoy to mob two public transport buses blocking the route, forcing drivers to rescue the vehicles from being set alight, as a large police presence melted away.

His SUV inched through the streets and down The Mall of Lahore, in a convoy of security vans, accompanied by private guards and supporters perched in vehicles and streaming behind on foot, said an AFP reporter.

‘We tried our best to stop the crowd but they did not stop,’ Lahore city police chief Habib-ur Rehman told AFP.

Nasir Zaidi, an intelligence official in Lahore, estimated that around 10,000 people were demonstrating in Lahore – 3,000 outside the high court and the rest thronging behind Sharif, heading towards the court.

He was later Sunday to address supporters in the city.

Facing the worst political crisis of his rule, Zardari has ordered a countrywide crackdown, banning protests, forcibly detaining activists and blocking provincial borders in a move that provoked concern in the West.

Soldiers armed with guns have sealed off the main entry into Islamabad from the garrison city of Rawalpindi, an AFP correspondent said.

Around 1,800 activists have been arrested since Thursday, the vast majority in Sharif's stronghold of Punjab province where the chief lawyer fighting to reinstate the judges, Aitzaz Ahsan, was also detained, officials said.

Analysts warned that a reluctant military, which has ruled Pakistan for more than half its 62-year existence, would be forced to intervene.

‘The situation is getting chaotic. It seems violence will take over and compel the army to intervene at some stage,’ defence and political analyst Talat Masood told AFP.

‘The army is extremely hesitant. But it is giving Zardari a firm message to come to terms with the opposition to avert violence,’ he told AFP.

The president late Saturday, held out concessions, vowing to appeal the court ruling that barred Sharif from office and pledging to restore the judges – albeit without providing any dates or firm details.