Supporters of the government and protesters loyal to opposition leader Imran Khan clashed on Friday during the second day of a march designed to force the country's prime minister to resign and call new elections.
The clashes are sure to heighten tensions ahead of the rally's arrival in the capital of Islamabad. Already the protest movement represents the biggest challenge to Prime Minister Nawaz Sharif's year-old government, which has virtually shut down the capital in anticipation of the rally's arrival.
There were conflicting reports of whether someone in the crowd had started shooting.
The protesters, who left the eastern city of Lahore on Thursday, are expected to arrive in Islamabad late Friday where they have vowed to camp out until their demands for government change are met.
Two separate rallies are bearing down on the capital. One is led by Khan, a cricket star who now heads the third largest party in parliament. The second is led by a fiery anti-Taliban cleric, Tahir-ul Qadri, who normally lives in Canada.
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 travelling with both Qadri and Khan although those numbers could swell when they reach the outskirts of Islamabad.
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 the demonstrators they would be dealt with "an iron hand" if they try to disrupt law and order.
Khan's convoy was in the city of Gujranwala when dozens of Sharif supporters pelted it with stones and shoes, Khan said. He was unharmed.
Video footage on Pakistani television showed supporters of Sharif's Pakistan Muslim League-N pelting the convoy with stones in Gujranwala, about 70 kilometres from Lahore. Rana Sanaullah, a PML-N leader, told the Dawn news channel that both sides threw stones at each other.
A police officer in Gujranwala, Mohammed Azeem, said about 200 supporters from the ruling party briefly clashed with Khan's supporters.
"The situation is under control," he said.
Khan told SAMA news channel that a supporter of his party, Sheikh Rashid Ahmed, told him he had seen a man firing in the air. Ahmed, a lawmaker from Rawalpindi and a close ally of Khan's, told the channel that he had seen someone firing but police said there were no shots fired. An Associated Press crew travelling with the convoy said they had not heard any shots.
The twin protest movements represent the toughest challenge yet for Sharif, who won his third term for prime minister during a landslide victory in May 2013.
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 has handed over power to another.
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 Baluch separatist groups have for years waged a low-level insurgency in Baluchistan.