They also blocked main road for traffic, demanding to hang the culprit in public so that no such incident will happen in future.
The protest was led by PPP MPA Saeed Ghani, Javed Nagori, Nadia Gabol and Waqar Mehdi.
A spokesman for the Punjab provincial police said paramilitary troops had been called in to restore order.
Candles were lit in memory of the minor.
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Zainab was the eighth minor to have been raped and murdered in Kasur in the past year, the police official said, adding that investigators could not confirm if they had a serial killer on their hands.
On January 10, protests had erupted in Kasur after the child's body was found in a garbage heap.
Reacting to the death of his daughter upon arrival with his wife from Saudi Arabia where they performed Umrah (holy pilgrimage) Ameen Asari, the father of the girl, claimed that the police "didn't do anything" after his relatives reported the disappearance of his daughter.
Zainab's mother, standing behind Ansari outside the airport, said: "I don't know anything. We do not want such an inadequate system".
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"It is true that a child's corpse is the heaviest".
Zainab's death comes less than three years after a scandal in Kasur when at least 280 children were filmed being sexually abused by a gang of 25 men, who blackmailed their parents by threatening to leak the videos. We have to act swiftly to punish the guilty & ensure that our children are better protected'.
Ruling PML-N member Punjab Assembly Hina Butt submitted a privilege motion in the assembly, seeking justice for 11 other minor girls who were killed after rape in Kasur and adjoining areas during the past year.
According to Daily Times, this is said to be the fourth case that has reportedly come to light in the past two weeks.
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"Finding Zainab's killers must be an important priority for authorities, especially as in this case it seems that this is not a lone incident in this area and other children are at risk", activist Maria Rashid noted. "There is no long-term redressal of the issue and it is treated more as an anomaly rather than a widespread problem that needs to be tackled at various levels and through sustained effort", she stressed.