Media playback is unsupported on your device Media caption The rape and murder of six-year-old Zainab sparked outrage in Pakistan.

The rape and murder of Zainab, a six-year-old girl from Pakistan, has sparked an outpouring of rage and grief online.

While the story has triggered angry demonstrations on the streets, more than half a million social media users have rallied around the hashtag #JusticeForZainab to call for action to be taken by the authorities, and to discuss the wider problem of sexual assault and the murder of children.

The body of the young girl was found in a rubbish dump in the city of Kasur, in the Punjab province, several days after her disappearance.

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At time of writing, #JusticeForZainab has been used over 600,000 times on Twitter since 9 January and variations of the hashtag have been used more than 30,000 times on public Instagram posts in the same period.

The hashtag began trending around the world as the news of the attack came to light, and received more attention after two protesters were killed by police.

While much of the reaction expresses horror and sadness, the origins of the hashtag seem to be political.

#JusticeForZainab was first used by activists for the opposition party Pakistan Awami Tehreek (PAT). The party is fiercely opposed to the ruling Pakistan Muslim League who control the Punjab province.

PAT campaigner @AtiqRehmanPAT was among the first to tweet the hashtag at 16:33 GMT on 9 January, saying the girl was "brutally abused and then silenced forever."

The hashtag has since been used by many political activists to deride the perceived failings of the local authorities, given the spate of abductions, sexual assaults and killings in the area, which have included 12 similar murders in the past two years.

Some used the hashtag to highlight other cases of sexual assault and murder against young children that have not received the same level of attention.

Zainab's parents have been willing to engage with the media and the growth of the hashtag has been propelled by well-known figures posting about Zainab.

Analysis: Secunder Kermani in Kasur, Pakistan

Image copyright EPA

There's still a lot of anger in Kasur. Protesters today ransacked the offices of a local politician and hurled stones at the police who fired tear gas in return - all just metres away from the rubbish dump where Zainab's body was discovered.

Zainab's father has received a steady stream of politicians at his home, promising he will receive justice.

He says he believes the killer will now be caught, but is angry with the police. He believes they didn't take the case seriously enough when his daughter first went missing.

Another father in Kasur, Asif Baba, shares his pain. His five-year-old daughter Ayesha disappeared in January last year. She too was murdered. Asif believes the same man who killed Zainab was responsible and he too blames the police for not investigating the killings properly.

Asif has kept his daughter's school uniform and doll collection in the room he shared with her. His eyes fill with tears as he shows them to me. He says when he heard about Zainab's murder he felt like he had lost his daughter all over again.

A number of Pakistani celebrities expressed their sadness over the incident, including professional cricketer Shadab Khan who said, "Zainab's tragedy is humanity's tragedy."

The story has made an impact around the world, with social media users in various countries discussing the death of Zainab.

Prominent human rights campaigner and Nobel Peace Prize winner Malala Yousafzai took to Twitter to call for the authorities to take action. Her tweet is the most retweeted use of #JusticeForZainab.

The hashtag has also started conversations around the role of gender in Pakistan. Karachi-based journalist Faizan Lakhani suggested people reflect on the upbringing of young boys.

The Muslim Women's Network, based in the UK, argued: "To truly achieve a world free from abuse, we need to tackle all forms of patriarchal/misogynist attitudes & promote equality/ justice through positive actions."

Many of the posts include a photo of the seven-year-old wearing a pink jacket and sparkly yellow jumper. However, some on Instagram are posting photographs of themselves with tape over their mouths and signs demanding justice for Zainab.