(CNN) -- Protesters in Pakistan held a sit-in near the country's main port Sunday, demanding an end to U.S. drone strikes.

Thousands of demonstrators gathered for the second consecutive day at a bridge near Karachi port, according to Pakistan's Tehrik-e-Insaf, or movement for justice.

CNN affiliate GEO TV showed protesters carrying signs and banners that said, "Stop drone strikes in Pakistan" and "Death to America."

Their aim is to halt work and stop trucks from leaving the port, a key supply hub for NATO troops in Afghanistan.

Last month Imran Khan, a famous former cricket player and the head of the right-wing group, threatened to have supporters block NATO supply routes to Afghanistan and march to Islamabad if U.S. drone strikes in the country were not halted.

Khan's party does not have any representation in parliament, but is popular among Pakistan's youth.

Thousands of people attended a sit-in in Peshawar, Pakistan, last month, blocking one of the routes used by NATO to send supplies from Pakistan to Afghanistan.

NATO supplies are transported from the port in the southern city of Karachi to Afghanistan through two routes.

The U.S. does not officially acknowledge the Central Intelligence Agency's secret drone program in Pakistan, but it is the only country operating in the region with the capacity to carry out such strikes. It was once widely believed that the program had the tacit approval and cooperation of Pakistan's intelligence agency.

In March, 44 people were killed in a drone strike in Pakistan's tribal region and the government of Pakistan formally asked the U.S. government for an apology.

After that strike, frequency of the drone strikes was reduced.