(Updates with quotes, court case)
By Katharine Houreld and Syed Raza Hassan
ISLAMABAD, Aug 16 (Reuters) - Tens of thousands of anti-government protesters flooded the centre of Pakistan's capital on Saturday, vowing to stay in the streets until Prime Minister Nawaz Sharif resigns.
The numbers were far below what protest organisers expected, but the protesters' paralysis of the central business district is presenting the 15-month-old civilian government with its biggest challenge yet.
The unrest raises 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 the influence of anti-Western and sectarian groups is growing.
Riot police cordoned off two streets in downtown Islamabad with shipping containers and barbed wire for the protests. Police estimated the numbers at about 60,000 for both marches combined, although Reuters reporters saw fewer.
Protest organisers say they are peaceful but determined.
"The prime minister and chief minister of Punjab should immediately resign and they should be sent to jail," thundered populist cleric Tahir ul-Qadri after arriving in a convoy of black Toyotas, their tinted windows speckled with rose petals.
"The participants of the Revolution March will not go from here till the resignation of Nawaz Sharif and Shahbaz Sharif, the dissolution of the government and their subsequent arrest," he said.
He spoke from a makeshift wooden pulpit atop a shipping container, with two men in black crouched on either side of him carrying bullet-proof blankets.
His supporters are adamant they will not leave until Qadri tells them to. The men carried stout sticks a few feet long. Brigades of men and women in fluorescent jackets had gas masks, swimming goggles and bottles of water.
"We are here to disarm the gas shells," explained one.
Qadri, a cleric and political activist who usually lives in Canada, controls a network of schools and Islamic charities.
He wants the prime minister to resign and a new government of technocrats installed. He promises he will crack down on corruption, and generate enough funds to pay for homes, jobs, cheap energy and water.
"Once corruption is eradicated, the country can move forward. It is the only thing holding us back," said 15-year-old Umme Habiba, a vivacious student with dimples and long black robes who said she came with her whole family.
Qadri is also protesting over the deaths of 14 of his supporters killed in clashes with police in June. He tried to register a case accusing Sharif of the deaths, but police had refused to take his complaint. On Saturday, the High Court in the eastern city of Lahore ordered his complaint recorded.
Former cricket star Imran Khan, who heads the opposition Pakistan Tehreek-e-Insaaf party, was also holding a smaller protest sit-in on an adjacent street.
The crowds - mostly young men - danced to music blasting from speakers or swapped T-shirts with Khan's face on them.
"Imran Khan is not a corrupt person. He's loyal to people and the country," said 25-year-old student Aqsa Ijaz, as she sat atop of a shipping container with her cousin, a banker.
Khan also wants Sharif to step down, accusing him of rigging last year's elections. Sharif won by a landslide, taking 190 out of 342 seats. Khan also did well in the elections, coming from political obscurity to take 34 seats, the third largest bloc in the legislature. But he says he should have had many more.
Khan waited until early evening, after it had begun to rain, to address his thousands of supporters. "I am restraining (my supporters) now but may not be able to soon," he said.
Like Qadri, Khan also commands intense personal loyalty from his followers. Pakistanis remember he led them to victory in the 1992 World Cup and his charity work is well-known.
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.
The government has annoyed the military by prosecuting former army chief and president Pervez Musharraf for treason and by insisting on months of fruitless peace talks before launching an anti-Taliban offensive.
The government is also struggling to overcome power shortages, high unemployment and spiralling crime - the main complaints of people at both protests.
"This government has failed the poor people," said farmer Razwan Baloch. "Everyone is here because life is so hard." (Additional reporting by Mehreen Zahra-Malik; Writing by John Chalmers and Katharine Houreld; Editing by Richard Borsuk and Stephen Powell)
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