The Pakistani capital braced for political unrest Thursday as two opposition leaders vowed to defy a court order and march on the city for a major protest aimed at bringing down the government.

Cricketer-turned-politician Imran Khan and populist preacher Tahir-ul-Qadri plan to lead thousands of supporters into Islamabad over allegations of rigging in last year's general election.
The protests could destabilise the government of Prime Minister Nawaz Sharif, who won a landslide victory in May 2013 polls, though the authorities have taken significant measures to try to thwart the planned protest marches, which are due to depart from the eastern city of Lahore.
More than 20,000 police and security forces have been deployed around Islamabad and almost all roads into the city have been blocked by the authorities with barbed wire and shipping containers.
Both Khan and Qadri say the 2013 general election was rigged and insist they will rally on Thursday, Pakistan's independence day, to demand that Sharif resigns and holds new polls.
Late on Wednesday the Lahore High Court barred Khan and Qadri's movements from marching on the capital "in an unconstitutional way".
Interior minister Chaudhry Nisar Ali Khan said the court order would be "fully implemented", saying any group who wanted to hold a demonstration had to get permission from the authorities.
But both Khan and Qadri said they would press on with their marches, Qadri warning the government would be "solely responsible" if violence occurred.
Khan, a cricket hero who led Pakistan to World Cup glory in 1992, has persistently cried foul over last year's election and tried numerous avenues to have the results of a number of seats thrown out.
But international observers who monitored the polls said they were free and credible and critics of Khan say his Pakistan Tehreek-e-Insaaf (PTI) party should not have accepted their seats in parliament if they did not believe the vote was fair.
- Vote-rigging probe -
Tension has gripped parts of the country since last week with running clashes between police and Qadri's supporters in Lahore over several days leaving at least one protester dead.
Some have accused the groups of being aided by the powerful military establishment to undermine the government, diverting attention from more pressing issues like an offensive against the Taliban in the northwest and the country's economy.
On Monday Qadri told AFP he wanted an "interim national government" consisting of technocrats and experts.
Sharif attempted to defuse some of the tension with a television address on Tuesday evening in which he announced a Supreme Court probe into vote-rigging allegations.
But the proposal was almost immediately rejected by Khan, who heads the country's third largest party and has called the 2013 election the most fraudulent in the country's history.
Political analyst and author Imtiaz Gul said the protests were one of the biggest challenges Sharif has faced since being elected for a third stint in office last year.
In a country which has seen three coups in its 67-year history, the threat of army intervention always hovers in the background at times of unrest.
But while Gul said there was a "real danger of bloodshed and violence", he added a coup was unlikely, though the coming days could leave the civilian government weakened.
"Based on the past experiences and in view of the the current national and international environment, the army will not possibly intervene," he said.
"However it may still use its political clout to nudge the politicians away from confrontation."
The military is rumoured to be unhappy with the way the Sharif government has pursued criminal charges, particularly a treason case, against former army chief Pervez Musharraf.
The former general returned to Pakistan last year in a bid to re-enter politics that went disastrously wrong as he was hit with a series of criminal cases dating back to his 1999-2008 rule.
Critics of Qadri, who lives in Canada much of the time, say that despite his image as a religious moderate, he is backed by elements within the powerful military establishment.
A four-day sit-in protest he led in central Islamabad in January last year, aimed at bringing down the then-government and getting a caretaker administration to enact reforms, sparked fears of a plot to derail the general election.
But his supporters see him as the only solution to a broken political system.