Leaders of two anti-government rallies today vowed not to back down till Prime Minister Nawaz Sharif resigns as thousands of their supporters converged on Pakistan's capital, posing the biggest challenge to the 15-month-old civilian government.

The opposition groups, led by cricketer-turned-politician Imran Khan and Canada-based cleric Tahirul Qadri, are protesting to press Sharif to call fresh polls alleging rigging during last year's elections.

"Under no circumstances, we will accept this (last) election. I am sitting here, Nawaz has one option. Resign and order a re-election," Khan, who heads the opposition Pakistan Tehreek-e-Insaaf party, said. "I will not leave here until I have got real freedom for the country," Khan said, as he addressed his supporters amid rains.

"I am not derailing democracy because there is no democracy in the country," he said.The protesters reached here from Lahore after travelling more than 300 kms in over 35 hours.

Khan announced that he will stage a sit-in from 3 pm today till Sharif resigns. Senior leaders of his party and workers also echoed similar sentiments and chanted "Go Nawaz Go" slogans.

The rain and journey took toll on Khan's health who had not slept for 40 hours, his supporters said.

According to Pervez Khattak, his party's chief minister in Khyber-Pakhtunkhwa province, Khan was suffering from high fever. He asked party workers to pray for Khan's quick recovery. Meanwhile, thousands of supporters of Qadri's "Revolution March" also reached at a separate venue in Islamabad.

"The whole thing will be very peaceful. The government has to resign, and the assemblies have to be dissolved and the new system has to take their place," Qadri told media ahead of his arrival in the capital.

He could not address his supporters after reaching Islamabad due to his poor health and will meet them later in the day. Qazi Faiz, spokesman of Qadri, said that the cleric was suffering from a sour throat but will address workers.

The political instability comes at a time when Pakistan is waging a war against militants -- particularly in the restive tribal regions along its border with Afghanistan.

Pakistan Supreme Court yesterday issued an order against any unconstitutional step to remove the civilian government as protests threatened to remove government which sparked fears of a possible military intervention in the coup-prone country. Both Khan and Qadri had initially planned to take out a joint rally to oust Sharif but later differences cropped up between the two.

Khan initiated his 'Azadi March' from his Zaman Park residence in Lahore, while Qadri launched his 'Revolution March' from the Model Town area of the city.

Government expects the marchers disperse peacefully as backdoor efforts are already going on to meet some of the demands of Khan and Qadri, who are now separately protesting against alleged rigging of last year polls.

Yesterday, as the march led by Khan passed through the city of Gujranwala, clashes erupted between his convoy and supporters of Sharif's Pakistan Muslim League-N after shots were fired at Khan's vehicle.

Sharif, on last Tuesday, had announced appointment of a panel of three judges of the Supreme Court to probe the charges of fraud in the elections. The government has deployed thousands of police and paramilitary soldiers to maintain law and order.