Over 25,000 supporters of Opposition leader Imran Khan and cleric Tahir-ul-Qadri vowing not to back down till Prime Minister Nawaz Sharif resigns and dissolves Parliament and provincial assemblies, poured into the streets of Pakistan's capital, posing the biggest challenge to the 15-month-old civilian government.

Unfazed by the country's apex court's order against any unconstitutional step to topple the government, the two Opposition groups held separate sit-ins, demanding Sharif's resignation and fresh polls alleging rigging during the last year's elections.

"The time has arrived when the nation should decide. I will stay here until the Prime Minister resigns. We don't accept a Prime Minister who has been appointed after rigged elections," Khan, who heads the Pakistan Tehreek-e-Insaaf (PTI) told his supporters after arriving here in a convoy, travelling over 300 km from Lahore for his 'Azadi March'.