By Syed Raza Hassan
ISLAMABAD (Reuters) - Clashes broke out Friday as tens of thousands of Pakistani protesters from two anti-government movements slowly converged on the capital, presenting the 15-month-old civilian government with its biggest challenge yet.
The unrest has raised questions over stability at a time when the nuclear-armed nation of 180 million is waging an offensive against Pakistani Taliban militants and the influence of anti-Western and sectarian groups is growing.
A stone-throwing mob attacked the convoy of former cricket star and opposition politician Imran Khan as he led supporters through the eastern city of Gujranwala. Men brandishing ruling-party posters attacked his convoy, throwing shoes and stones.
Khan's convoy was shot at but he was not injured, his spokeswoman said. The government insisted shots were not fired and promised an investigation into the incident.
"The Chief Minister of Punjab has ordered an inquiry and all those responsible for scuffle will be held accountable," the statement said. "There were absolutely no gunshots fired at his rally and such PTI-driven sensationalism is unfortunate."
Khan's Pakistan Tehreek-e-Insaaf party and supporters of populist cleric Tahir ul-Qadri are slowly heading towards Islamabad, where they plan to occupy main streets until Prime Minister Nawaz Sharif resigns.
In the capital, authorities blocked main roads with shipping containers and barbed wire in a effort to control the marches.
Riot police were out in force but hundreds of protesters began to gather, banging drums, singing and dancing as they prepared to welcome their comrades approaching the city.
"We have come to save our country because of the call of our leader, Imran Khan," said 36-year-old Ajaz Khan in central Islamabad. "We will not leave from here until our leader tells us to go."
In the latest violence, 10 militants were killed and 13 members of the security forces were wounded in attacks on two air force bases in the city of Quetta late on Thursday, the third time since June airports had been targeted.
Some members of Sharif's party have suggested the protests are secretly backed by elements in the powerful military, which has had an uneasy relationship with Sharif.
How far Khan and Qadri succeed in destabilising the government is likely to depend on the stance taken by a military, which has a long history of mounting coups.
Few people fear a coup but many officials think the threat of unrest will increase the military's hold over the government.
The military has been frustrated with the government, in particular over the prosecution of former army chief and president Pervez Musharraf for treason.
There has been disagreement too between the government and the army on how to handle the Taliban. The army favours military action but the government insists on peace talks.
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The government is further struggling to overcome daily power shortages, high unemployment and spiralling crime - the legacy of decades of corruption and neglect.
Anger over the economy means the protests appeal to many disillusioned young Pakistanis.
Both protest leaders also command intense personal loyalty from their followers. Khan is a famed former international cricketer, known for his charity work, who now heads the third largest legislative bloc in the country.
Qadri, a cleric and political activist who usually lives in Canada, controls a large network of schools and Islamic charities. His followers intend to occupy Jinnah Avenue, Islamabad's main thoroughfare.
"We will not go back until Sharif resigns," said Qadri's spokesman, Shahid Mursaleen. "They killed our people, there is no way we can make a deal with them."
Qadri has accused police of killing 22 of his supporters during clashes in the eastern city of Lahore in June and this month. Police confirmed 11 deaths. About 2,000 of Qadri's supporters were also arrested this month, police said.
Khan is protesting against alleged electoral irregularities in last year's voting.
Most observers expect the military to play referee - to maintain security but not support action to force Sharif out.
"Imran will not get from the army what he was expecting," said an analyst close to the military.
"If there was any confusion earlier about whether the army would help Imran or rescue him or topple the government, there should be none now. There is no question of army intervention."
(Additional reporting by Katharine Houreld and Mehreen Zahra-Malik; Writing by Katharine Houreld; Editing by Mark Heinrich)