ISLAMABAD, Pakistan — At first, the killing last month of Naqeebullah Mehsud — an aspiring model shot by the police in Karachi who claimed afterward that he was a Taliban militant — seemed merely the latest in a long series of abuses carried out by the authorities against ethnic Pashtuns in Pakistan.

But Mr. Mehsud’s case has proved different. The 27-year-old’s killing, in what appears to have been a staged gun battle, has prompted a protest movement led by young Pashtuns from the tribal areas in the country’s northwest, where they have long been the targets of military operations, internal displacement, ethnic stereotyping and abductions by the security forces.

Last week, a social-media-savvy group of young Pashtuns organized a sit-in in Islamabad, the capital, promoting it with the hashtag #PashtunLongMarch. As of Tuesday, the demonstration’s sixth day, at least 5,000 Pashtuns from the tribal areas and other parts of the country had joined, and members of all major Pakistani political parties had declared their support.

“Certainly, this kind of organized struggle for Pashtun rights, reforms and resources has not been seen in years and years,” said Rahimullah Yusufzai, the Peshawar-based editor of The News, a Pakistani newspaper. “The people of the tribal areas have had pent-up feelings of resentment and anger at their treatment by the state for decades,” he added. “Naqeebullah’s killing was just the tipping point.”