ISLAMABAD, Pakistan -- Tens of thousands of anti-government protesters gathered in Pakistan's capital Islamabad late Friday following the arrival of convoys led by a cricket star-turned-politician and a fiery anti-Taliban cleric.

The twin protests led by Imran Khan and the cleric Tahir-ul-Qadri represent the biggest challenge yet to Prime Minister Nawaz Sharif's year-old government and security has been tightened across the capital amid fears of unrest in a country with a long history of chaotic politics and military coups.

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.

They remained on the road for two days before entering Islamabad shortly before midnight. Police estimated the crowd at 60,000 people.

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

Sharif says he is 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.

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 kilometres) 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 one million of their followers into the streets of Islamabad, a city of roughly 1.7 million inhabitants.

Thousands of riot police have been deployed across the capital. Authorities set up shipping containers to block traffic and cut off cellphone service in some areas.

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 twin protest movements represent the toughest challenge yet for Sharif, who won a landslide election victory in May 2013. Khan 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 much influence over life in Pakistan, which is battling several militant groups.

Late Thursday, attackers tried to storm two air bases in the southwestern city of Quetta, sparking a gun battle that killed 10 militants, police said.

Police chief Muhammad Amlish said seven security personnel were wounded in the attack. He said the attackers used guns and grenades as they tried to enter the Smungli and Khalid military bases on a sprawling complex next to the city's airport.

The army said 11 "terrorists" were killed in the attack and another three apprehended. Initial police reports had said only two attackers were involved.

Pakistan's army chief Gen. Raheel Sharif said militants were "desperate" and on the run amid a massive military operation in North Waziristan, a tribal region that has long been home to various insurgent groups.

Hours after the attack, Azam Tariq, a spokesman for the Pakistani Taliban, claimed responsibility. Inspired by the Afghanistan Taliban across the border, the group wants to overthrow the government in Islamabad and impose its harsh interpretation of Islamic law.

In a statement, he said Uzbek and Pakistani militants carried out the attack in response to the offensive in North Waziristan.