Thousands of members of political and religious parties have begun a march toward Pakistan's capital in a massive convoy of vehicles to protest against the government's decision to allow the US and other NATO countries to resume shipping troop supplies to Afghanistan.
The "long march", which started in the eastern city of Lahore, was organised by the Defence of Pakistan Council, a group of more than 40 political and religious parties who have been the most vocal opponents of the supply line.
Pakistan closed the route in November in retaliation for US airstrikes that killed 24 Pakistani troops.
Following months of negotiations, Islamabad finally agreed to reopen the route last week after Hillary Clinton, the US secretary of state, apologised for the deaths.
Clinton met with Pakistani Foreign Minister Hina Rabbani Khar for the first time since the apology on Sunday on the sidelines of an Afghan aid conference in Tokyo and expressed hope that resolution of the supply line conflict would lead to better relations between the troubled allies.
One of the reasons Pakistan waited so long to resolve the conflict is that the government was worried about a domestic backlash in a country where anti-US sentiment is rampant despite billions of dollars in US aid over the past decade.
The US drone campaign in northwest Pakistan, which has killed thousands of people since 2004, many of them civilians, remains a huge source of anger.
'Hatred for America'
The protest started on Sunday in the centre of Lahore, where several thousand people assembled with scores of buses, cars and motorbikes.
They linked up with thousands more supporters waiting on the city's edge and drove toward Islamabad.
The convoy included about 200 vehicles carrying about 8,000 people when it left Lahore, said police official Babar Bakht.
After completing the 300km journey to Islamabad, the marchers plan to hold a protest in front of the parliament building on Monday.
"By coming out on the streets, the Pakistani nation has shown its hatred for America," one council leader, Maulana Samiul Haq, said in a speech on the outskirts of Lahore.
Supporters showered Haq with rose petals as he rode through Lahore in the back of a truck with other council leaders, including Hafiz Saeed, founder of the banned Lashkar-e-Taiba group; Hamid Gul, a retired Pakistani intelligence chief; and Syed Munawar Hasan, leader of Pakistan's most powerful religious party, Jamaat-e-Islami.
Many demonstrators rode on the tops of buses, waving party flags and shouting slogans against the US and NATO.
"One solution for America, jihad, jihad!" they shouted.
'The mission is noble'
The crowd was dominated by members of Jamaat-ud-Dawa, widely believed to be a front group for Lashkar-e-Taiba, which is blamed for the attacks in the Indian city of Mumbai in 2008 that killed more than 160 people.
Jamaat-ud-Dawa is led by the group's founder, Saeed.
"The movement that has been started to reverse the government's decision to restore the NATO supply will go on until America leaves this region for good," Saeed said in a speech on the outskirts of Lahore.
"The mission is noble because it is to save the country and the nation from slavery."
The US announced a $10m bounty earlier this year for information leading to the arrest or conviction of Saeed, but he operates freely in the country.
Pakistan says it does not have enough evidence to arrest Saeed, but many suspect that the government is reluctant to move against him and other leaders because they have alleged longstanding ties with the country's military and intelligence service.
Rehman Malik, a government security adviser, said members of banned groups would not be allowed to enter Islamabad for the protest on Monday, but all others would be welcomed.
"They are patriots. They are not anti-state people," Malik told reporters. "We will welcome them with open arms."
It is unclear if they will try to prevent Saeed from attending the protest.
The Defence of Pakistan Council is widely believed to be supported by the Pakistani army as a way to put pressure on the US.
Its leaders have vowed to stop NATO trucks from making the journey from the southern port city of Karachi to the Afghan border. But if the group has army backing, it could moderate its actions.
Although the army was outraged by the US attack on its troops, which Washington said was an accident, it was eager to repair the relationship to free up more than $1bn in military aid that had been frozen for the past year.
The US waited so long to apologise in part because the Obama administration was apparently worried such a move would expose it to criticism from Republicans in a presidential election year.
Many US officials and politicians harbour deep suspicions of Pakistan, citing the country's alleged support for groups fighting US troops in neighbouring Afghanistan.
While the supply line through Pakistan was closed, the US was forced to rely on a longer, more costly route that runs into Afghanistan through Central Asia. The route cost an extra $100m per month.
The Obama administration also wanted to resolve the conflict because it needs Pakistan's help to strike a peace deal with the Taliban in Afghanistan so that US troops can withdraw without the country descending into further chaos.
Pakistan is seen as key to an agreement because of its strong historical ties with the Taliban and its allies.