Zainab was kidnapped six days ago from Road Kot when her parents left for Saudi Arabia to perform umrah.
Protests sparked by her death turned violent with a mob throwing stones and some shooting bullets at a local police office.
Nobel Peace prize victor Malala Yousafzai, condemned the murder of Zainab, saying "such brutal incidents have to stop".
"We provided them CCTV footage of the kidnapping incident to the police, but they did not pay much attention to it", Zainab's uncle said.
People hold bamboo sticks as they stage a protest against Zainab's murder.
Lawyers boycotted judicial proceedings at district bar in protest and demanded early action into the matter.
The distraught father of the 7-year- old girl has blamed the "callous attitude" of police as the government roped in military intelligence and the ISI to probe the horrific incident that shocked the country.
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A number of police officials have been transferred out of the region for failing to investigate complaints of missing children since 2015, when authorities uncovered what they called a paedophile ring linked to a prominent local family.
Zainab Ansari was last seen on her way to a religious recital when her body was found 2km from her home.
Punjab's Chief Minister Muhammad Shehbaz Sharif ordered police to arrest the girl's killers in the next 24 hours and offered a 10 million Pakistani rupee bounty (over $90,000) for anyone who helps identify those responsible.
Zainab was one of several minors to be raped and killed in or near Kasur in the past year, Sky News reported.
In a condemnation posted on Twitter, PTI chief stated: "This is not the first time such horrific acts have happened".
The Govt and Concerned security institutions have to act swiftly to punish the guilty and ensure that children are better protected and the government must be mindful of its responsibility to protect the life, honour and property of the masses.