ISLAMABAD — Supporters of the Pakistani government and opposition protesters clashed on Friday during the second day of a march to the capital aimed at forcing Prime Minister Nawaz Sharif to resign.

The clashes raised tensions ahead of the protesters' arrival in Islamabad, which has been on a virtual lockdown in recent days. The twin protests led by famed cricketer-turned-politician Imran Khan and anti-Taliban cleric Tahir-ul-Qadri represent the biggest challenge yet to Sharif's year-old government.

The protesters left the eastern city of Lahore on Thursday and are expected to arrive in Islamabad late Friday, where they have vowed to camp out until their demands for a new government are met.

As the march led by Khan passed through the city of Gujranwala on Friday, 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."

Sheikh Rashid Ahmed, a lawmaker and close ally of Khan, told the SAMA news channel that he had seen someone firing a gun, but police said no shots were fired. An Associated Press crew traveling with the convoy said they had not heard any gunshots.

Both Khan and Qadri have said they will draw one million of their followers into the streets of Islamabad, a city of roughly 1.7 million inhabitants. So far, a few thousand are estimated to be traveling with both marches, but those numbers could swell when they reach the outskirts of the capital.

Ahead of the rally, thousands of riot police were 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.

Associated Press Writer Abdul Sattar from Quetta contributed to this report.