Thousands of Islamists are traveling from Lahore to Islamabad in Pakistan for a planned march to protest the reopening of NATO supply routes.
The effort is led by Hafiz Saeed, alleged mastermind of the 2008 terrorist attacks in Mumbai, India, that left 164 people dead.
Along with leaders of religious and right-wing political parties, Saeed is fighting the reopening of a route NATO uses to get supplies to Afghanistan.
Pakistan closed it last November after NATO fighter jets attacked a Pakistani checkpoint near the Afghanistan border, killing 24 Pakistani soldiers.
Officials in Pakistan reopened the route last week after U.S. Secretary of State Hillary Clinton apologized for the incident.
Hamid Gul, the so-called "father of the Taliban," and Syed Munawar Hassan, head of Pakistan's largest religious party Jamat-e-Islami are also leading the long march.
Most of the protesters appeared to be with Saeed's banned group Lashkar-e-Tayyiba, which the United States identifies as a terrorist group.
On Sunday, around 20,000 protesters could be seen traveling in more than 300 vehicles toward Islamabad.
The march is expected in Islamabad on Monday.
The United States offers up to $10 million for information leading to Saeed's arrest and conviction. But his whereabouts in Pakistan are no secret. "I am living my life in the open and the U.S. can contact me whenever they want," Saeed told Geo TV in Islamabad in April.
He said the Pakistani supreme court had cleared him and his organization of wrongdoing, and he condemned the Mumbai attacks.