Police found Zainab's body in a rubbish dump in the city of Kasur in the province of Punjab on Tuesday, a week after she was kidnapped, local police chief Zulfikar Ahmed said on Wednesday.
An autopsy suggested the girl was raped before being strangled to death at least five days ago, Ahmed said.
There have been a dozen similar incidents in Kasur in past year in which minor girls were kidnapped, raped and killed, Ahmed added.
"All the cases have resemblances. It looks a lunatic serial killer is behind these incidents," provincial government spokesman Malik Ahmed Ali said, citing an initial inquiry.
Seven-year-old Zainab was raped and murdered. Source: Twitter
A handout photo by Pakistani Awami Tehreek shows Tahirul Qadri (C), a religous scholar leads the funeral prayers of the seven-years-old girl Zainab. Source: AAP
In 2015, the city of Kasur made world global headlines when hundreds of videos of child molestation surfaced and police later arrested several members of a racket involved in selling child pornography to websites in Europe.
During the riots on Wednesday, angry demonstrators stormed the streets with two protesters killed after police shot at an angry crowd in Kasur, 20 km south of Lahore, according to the BBC.
Protesters demanded the immediate arrest of the culprit while chanting slogans against the authorities, as chaos erupted.
Amin Ansari, Zainab's father, said he will not bury his child until justice is served.
"I have just learned that police have fired at people, and two persons have been martyred," he said.
"I am going there to find out what happened. We are going to try and not bury our child until the issue has been properly resolved. We are going to appeal to the chief justice of Pakistan and to the army chief of Pakistan that everyone has daughters, and if there is any humanity, they should take suo motu action immediately."
The official spokesperson of the Pakistan Armed Forces responded to community anger by offering its support to arrest the culprit.
This was backed by the Government and Chief Minister Punjab Shehbaz Sharif who put together a Joint Investigation Team to submit a report within 24 hours.
The Government of Pakistan's official Twitter account also said the four policemen accused of firing into the crowd in Kasur have been arrested.
Hashtag 'Justice for Zainab' seeking justice for the child was trending in Pakistan, where thousands of people were expressing their anger and shock.
The latest incident has shocked the nation with political leaders, civil society activists, sportsmen and artists calling for action by the government to protect children and punish the perpetrators.
