According to local media reports, the girl was kidnapped while her parents where in Mecca performing a religious pilgrimage, and her body discovered by passers-by, in the town of Road Kot, in Pubjab province.
Speaking at Islamabad airport as he arrived back in the country, her father Ameen Ansari said, as his wife sobbed: 'We will not bury Zainab until the killer is caught.
'He alleged that the police did not cooperate in finding his daughter 'If the police would have acted immediately, the culprit would have been caught.'
Her mother added: 'I have nothing to say, I just want justice for my daughter.'
A mob angered over murder attacked a police station and a nearby government building in eastern Punjab province on Wednesday, triggering clashes that left at least two people dead and several injured, the police said.
The violence erupted in the city of Kasur hours before the funeral of Zainab and shopkeepers in the city closed their premises in support.
Much of the anger was directed Punjab Chief Minister Shehbaz Sharif and Punjab Inspector General of Police who they claimed had failed to act on dozens of cases of children being raped and murdered in the Kasur district of the province.
Former cricket star Imran Khan was among those who expressed their shock at the killing
A large number of women joined in similar protest in Punjab and elsewhere in Pakistan, it was reported.
Pakistanis expressed their shock on social media using the hashtag 'justiceforZainab'.
Among those voicing anger was the cricket star and politician Imran Khan who tweeted: 'The condemnable & horrific rape & murder of little Zainab exposes once again how vulnerable our children are in our society.