By Katharine Houreld and Syed Raza Hassan
ISLAMABAD (Reuters) - The leaders of two Pakistani protest movements vowed on Saturday to keep their protests alive until Prime Minister Nawaz Sharif resigns, after thousands entered the country's capital in the biggest challenge yet to the 15-month-old civilian government.
"I will not leave here until I have got real freedom for the country," former cricket star Imran Khan, who heads the opposition Pakistan Tehreek-e-Insaaf party, told thousands of supporters who followed him into Islamabad the day before.
"Decide Nawaz Sharif! Resign and announce elections."
Populist cleric Tahir ul-Qadri, who has spearheaded a separate protest march to the capital, planned to stage a sit-in with his followers and deliver a speech at around midday.
"Dr. Qadri will present the 'Demands of Revolution March 2014', which includes stepping down of Nawaz Sharif and his government and his immediate arrest," said Qadri's spokesman, Shahid Mursaleen.
The unrest has raised questions about Pakistan's stability, at a time when the nuclear-armed nation of 180 million is waging an offensive against Pakistani Taliban militants and when the influence of anti-Western and sectarian groups is growing.
On Friday, a stone-throwing mob attacked Khan's convoy as he led supporters through the eastern city of Gujranwala. Gunshots were fired but Khan was not injured, his spokeswoman said. The government said there was no shooting, and police arrested 16 activists from the ruling party.
In Islamabad, authorities have blocked roads with shipping containers and barbed wire in an effort to control the marches, and riot police have been out in force.
"We think Imran Khan will make a better country for the youth," said 25-year-old Muhammad Taraki at Khan's rally point. "I have a bachelor's degree, but I cannot get a job."
Some members of Sharif's party have suggested the protests are secretly backed by elements in the military, which has had an uneasy relationship with Sharif. To what extent Khan and Qadri can destabilize the government is likely to depend on the stance taken by the armed forces, 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 government insisted on peace talks but eventually the army launched an offensive.
MILITARY MAY AVOID TAKING SIDES
The government is also struggling to overcome 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.
Leaders of the two protests 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 Pakistan. He is protesting alleged irregularities in last year's voting.
Qadri, a cleric and political activist who usually lives in Canada, controls a network of schools and Islamic charities. His followers say they intend to occupy an area of Islamabad called Zero Point.
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 Mehreen Zahra-Malikin in ISLAMABAD and Asim Tanveer in MULTAN; Writing by John Chalmers; Editing by Richard Borsuk)