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K.M. Chaudary, Associated Press
One of the supporters of Pakistan's ruling party throws a rock on a procession of cricketer-turned-politician Imran Khan in Gujranwala, Pakistan, Friday, Aug. 15, 2014. Dozens of members of Pakistan’s ruling party threw stones and shoes at a truck carrying a popular opposition leader Khan Friday, as he was on the road for the second day to travel to the capital, Islamabad, for a rally meant to pressure the country’s Prime Minister Nawaz Sharif to resign over allegations of rigging last year’s parliamentary elections.

ISLAMABAD — Pakistan's famous cricketer-turned politician Imran Khan early Saturday asked Prime Minister Nawaz Sharif to step down to pave the way for fresh elections, saying last year's vote was massively rigged.

Addressing tens of thousands of supporters in the pouring rain in the capital, Islamabad, he vowed to continue a sit-in until Sharif quits.

Khan's demand came hours after he arrived in Islamabad after leading a big convoy of party workers.

The protesters left the eastern city of Lahore on Thursday, vowing to march to the capital and camp out there until their demands for a new government are met. Despite the darkness and the lashing rain, the crowds swelled as they entered Islamabad shortly before late Friday night.

An anti-Taliban cleric Tahir-ul-Qadri also reached Islamabad late Friday, and he too led tens of thousands of his supporters to bring about what he called a "green revolution." The twin protests led by Khan and Qadri represent the biggest challenge yet to Sharif's year-old government.

Security has been tightened across the capital amid fears of unrest in a country with a long history of chaotic politics and military coups. Authorities set up shipping containers to block traffic and cut off cellphone service in some areas.

Police estimate some 60,000 people were taking part in the rallies.

Khan told supporters he would again address them later Saturday.

The protests were festive despite the rain, with demonstrators waving national and party flags and dancing to drum beats and patriotic songs. Women supporters of Qadri, wearing Islamic headscarves, lined the roads and waved at his convoy as it entered the city.

As he approached the Islamabad airport, Khan tweeted that he would stage the sit-in on the city's main Kashmir Highway. "Sharif should have his resignation ready," he said.

A spokesman for Qadri, Shahid Mursaleen, said the cleric would also deliver a speech Saturday to call for Sharif's removal and immediate arrest.

Sharif says he was ready to meet with his opponents but has given no indication that he would step down. His critics accuse him of vote fraud during the election that brought him to power last year.

Sharif's spokesman, Pervaiz Rashid, condemned the "irresponsible behavior and actions" of his opponents.

"Pakistan is not a banana republic, where a few thousand people come and seek the resignation of the country's prime minister," he told a local news channel.

Earlier Friday, as the march led by Khan passed through the city of Gujranwala, supporters of Sharif's Pakistan Muslim League-N hurled stones at the convoy, said Khan, who was unharmed.

PML-N leader Rana Sanaullah told the Dawn news channel that both sides threw stones at each other.

Mohammed Azeem, a police officer in Gujranwala, about 40 miles (70 kilometers) from Lahore, said some 200 ruling party supporters clashed with Khan's protesters but that "the situation is under control."

Both Khan and Qadri have vowed to bring 1 million followers into the streets of Islamabad, a city of roughly 1.7 million inhabitants.

Interior Minister Chaudhry Nisar Ali Khan on Thursday apologized for the city's paralysis, insisting the measures were for the residents' own safety and warning that demonstrators would face an "an iron hand" if they try to disrupt law and order.

The protests represent the toughest challenge yet for Sharif, who won a landslide election victory in May 2013. Khan, who led Pakistan to victory in the 1992 cricket World Cup, heads the third largest party in parliament.

Pakistan, a nuclear-armed country of 180 million people, has largely been ruled by military dictators since it was carved out of India in 1947. Last year's election marked the first time that one elected civilian government had handed over power to another.

The army still wields great influence in Pakistan, which is battling several militant groups, but has not taken sides in the protests. There are fears, however, that political unrest could prompt the military to intervene.

Associated Press reporters K.M. Chaudhry and Muhammad Farooq in Islamabad and Abdul Sattar in Quetta contributed to this report.