ISLAMABAD (AP) - Thousands of Pakistani opposition supporters were on the road for a second day Friday, heading in two separate convoys to the capital, Islamabad, for a massive rally meant to pressure the country's prime minister to resign over allegations of rigging last year's parliamentary elections.

The convoys, which started out on Thursday morning from the city of Lahore, were in response to calls by two very different opposition figures: Imran Khan, the famous cricketer-turned-politician, and firebrand cleric Tahir-ul-Qadri who commands a strong following through his network of mosques and religious schools in Pakistan.

Both have challenged the government of Prime Minister Nawaz Sharif, who came to power in 2013 in the first democratic transfer of power in a country which has seen three coups since gaining independence in 1947.

They demand new elections under the supervision of a neutral government, but Sharif's aides say the demand is unconstitutional. Khan also wants a new election commission chief appointed before the vote is held, while Qadri says electoral reforms are necessary.

The opposition march comes at a time when Pakistan's military is fighting militants in the troubled North Waziristan tribal region, which has been a base for militants accused of launching attacks in Pakistan and neighboring Afghanistan. The military has killed over 500 militants there since launching the June 15 operation.

The government has criticized Khan and Qadri, saying the country needs unity - not turmoil and dissent - at a time when the armed forces are fighting militants who have killed thousands of people in recent years. Authorities have also said there have been intelligence reports about possible attacks on the convoys or the opposition rally, which is expected to start Friday evening in Islamabad.

After 20 hours on the road, the slow-moving convoys had covered about half of the distance of 300 kilometers (187 miles) toward the capital.

"Listen, Nawaz Sharif, I am coming to Islamabad to seek your resignation," a boisterous Khan told followers from atop his truck as it passed through the city of Gujranwala, about 200 kilometers (125 miles) away from Islamabad.

In Gujranwala, dozens of Sharif supporters pelted Khan's truck with shoes and stones but he was unharmed, his aides said. Pakistani TV showed supporters and Khan and Sharif throwing stones at each other in the city.

Earlier, Khan said he was hoping to lead a march of one million people in the Pakistani capital later in the day. Qadri also said he expected one million people to join him in Islamabad, a city of 1.7 million residents according to a 2012 census.

Ahead of the rally, thousands of riot police and special units were deployed across Islamabad. Authorities also blocked many roads in the capital with shipping containers.

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 the demonstrators they would be dealt with "an iron hand" if they try to disrupt law and order.

Sharif criticized his opponents for pursuing "negative politics" and promised to safeguard democratic institutions.

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.

Sharif has also been the victim of a military coup. His elected government was ousted in 1999 by then army chief Pervez Musharraf.

The army still wields much influence over life in Pakistan, which has seen frequent attacks by militants and insurgents of various backgrounds and agendas.

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

Police chief Muhammad Amlish said seven security personnel were also 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. Initial police reports had said only two attackers were involved.

No one claimed responsibility for the attack but nationalists groups have for years waged a low-insurgency in Baluchistan to pressure the government for a fairer share of local resources.

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