A Pakistani mob killed a female member of a religious minority and two of her granddaughters after another sect member was accused of posting blasphemous material on Facebook, police said Monday, the latest instance of growing violence against minorities in Pakistan.

The victims, including a 7-year-old girl and her baby sister, were Ahmadis, who consider themselves Muslim but believe in a prophet after Mohammed. A 1984 Pakistani law declared them non-Muslims and many Pakistanis consider them heretics.

Blasphemy against Islam is strictly forbidden by Pakistan law and punishable by death, and extrajudicial killings like Sunday's are also on the rise.

Police said the violence in the town of Gujranwala, 140 miles southeast of the capital, Islamabad, started with an altercation between young men, one of whom was an Ahmadi accused of posting "objectionable material."

A police official, Zeeshan Siddiqi, told the Associated Press that the image in question allegedly depicted the Kaaba — the cube-shaped structure in the Grand Mosque of Mecca, Saudi Arabia, which observant Muslims around the world face in prayer five times a day — and contained nudity.

"A crowd of 150 people came to the police station demanding the registration of a blasphemy case against the accused," one police officer, who declined to be identified, told Reuters. "As police were negotiating with the crowd, another mob attacked and started burning the houses of Ahmadis."

The three victims died of suffocation, police official Zeeshan Siddiqi said. The youth accused of making the Facebook post had not been injured. His relationship to the victims was not clear.

Pakistani TV channels aired footage showing a mob armed with sticks, cheering outside Ahmadi houses set on fire in Gujranwala. Resident Munawar Ahmed, 60, said he drove terrified neighbors to safety as the mob attacked.

"The attackers were looting and plundering, taking away fans and whatever valuables they could get hold of and dragging furniture into the road and setting fire to it… Some were continuously firing into the air," he said. "A lot of policemen arrived but they stayed on the sidelines and didn't intervene," he said.

Salim ud Din, a spokesman for the Ahmadi community, said it was the worst attack on the community since simultaneous attacks on Ahmadi places of worship killed 86 people four years ago.

Under Pakistani law, Ahmadis are banned from using Muslim greetings, saying Muslim prayers or referring to their place of worship as a mosque. They also are not allowed to refer to themselves as Muslims.

Accusations of blasphemy are skyrocketing in Pakistan, from one in 2011 to at least 100 already this year, according to the Human Rights Commission of Pakistan. Currently, there are at least 17 people convicted of blasphemy on death row in Pakistan, with another 19 serving life sentences.

But since 1990, at least 60 people have been killed outside the Pakistani justice system in cases relating to blasphemy, according to the Islamabad-based Centre Research and Security Studies (CRSS). The list includes lawyers, alleged blasphemers and even politicians calling for amendments to the law.

Human rights workers say the accusations are increasingly used to settle personal vendettas or to grab the property of the accused.

This climate of fear makes it difficult to prosecute blasphemy cases. Judges often hold court sessions inside jails because they are considered the only safe places for the proceedings. Witnesses are reluctant to testify in defense of people accused of blasphemy, fearing they could also be targeted.

In May, gunmen in the city of Multan killed a lawyer and a human rights activist who was representing a university professor on trial for blasphemy. The professor is also accused of posting blasphemous material at Facebook.

Al Jazeera and wire services