Protestors set fire to a bus and fought pitched battles with police
Pakistan opposition leader Nawaz Sharif has defied an apparent bid to put him under house arrest in Lahore ahead of a "march" on the capital Islamabad.
Thousands of supporters joined him after he broke through a police barricade of his home to reach a rally.
Police fired tear gas as protesters hurled stones.
Sharif activists later managed to overcome barriers blocking access to the main highway to Islamabad and Mr Sharif left Lahore in a convoy.
The Pakistan Muslim League (PML-N) supporters plan a "long march" to the capital to demand judges sacked by the former government be reinstated.
"It's now a matter of the future of Pakistani nation and coming generations," Mr Sharif said earlier by telephone from his bullet-proof car.
"How can we abandon our mission halfway?"
Mr Sharif is expected to arrive in Rawalpindi - near Islamabad -in the early hours of Monday. He is due to be joined there by his brother, Shahbaz Sharif, former chief minister of the Punjab, and leaders of the lawyers' movement.
Police are said to be surrounding the property in Rawalpindi where his brother is believed to be staying.
Sharif activists used mobile hydraulic lifts, apparently brought along for the purpose, to remove massive containers placed as obstacles on the highway.
The house arrest is illegal and immoral, all these decisions are unconstitutional
The BBC's Barbara Plett says it is not clear if Mr Sharif will be able to reach Islamabad, given the authorities have blocked routes leading to the capital.
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.
Party spokesman Ahsan Iqbal told the BBC: "A government which claims to be a democratic government is coming with such heavy-handed fascist tactics."
But interior ministry chief Rehman Malik told the BBC the police were outside Mr Sharif's home for his own protection because of the threat from terrorists.
Mr Sharif was ousted as prime minister in 1999 during a coup by General Pervez Musharraf, who ruled until August 2008.
Tensions between Mr Zardari and Mr Sharif date back to the 1990s, but the two formed a brief partnership in government after parliamentary elections in February 2008.
Mr Sharif's party later left the alliance, complaining of reluctance by Mr Zardari's Pakistan People's Party to reinstate the judges sacked by the last government.
Relations have been further strained in recent weeks by a Supreme Court decision to ban Mr Sharif and his brother Shahbaz from elected office, and President Zardari's decision to put their stronghold in Punjab province under direct rule from Islamabad.
But on Saturday, in a move seen as a conciliatory gesture, the government agreed to seek a review of the Supreme Court ruling.
The political instability comes as Pakistan faces an economic crisis and a growing militant insurgency based in the north-west.
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