In a statement, Human Rights Watch said that much of the violence had been carried out by the security forces and so-called war veterans loyal to Robert Mugabe, the president.
It said the unrest was centred on areas which had traditionally been Zanu-PF strongholds but in recent years have increasingly turned to the opposition.Civilians 'armed'
Tiseke Kasambala, a Human Rights Watch researcher who recently returned from Zimbabwe, told Al Jazeera that the group spoke to witnesses and victims who said the army was orchestrating the violence and arming the "war veterans".
"The fact that the military is arming civilians is of serious concern and if this becomes widespread there is no telling where Zimbabwe will go next," she said.
She called on the African Union and the UN to intervene to protect Zimbabweans.
"The international community can simply not afford to wait for such things to happen," Kasambala said.
On Tuesday, the Security Council failed to agreed on a common strategy on the situation in Zimbabwe.
South Africa, Russia and China were among the countries which blocked moves towards any UN intervention despite pleas by the MDC for a special UN envoy to be sent to the country.
Mugabe's government has repeatedly denied organising any violence, saying that the opposition groups are responsible.
Bright Matonga, Zimbabwe's deputy information minister, told the AFP news agency that the move was a plot by Britain, Zimbabwe's former colonial ruler, "to bully African nations".Polling official arrests
Human Rights Watch also said it had received reports that more than 100 polling station officers - most of them teachers and low-ranking civil servants - had been detained in an eastern province.
Kasambala described that as another indication the government and its loyalists were targeting those seen as betraying Mugabe.
Augustine Chihuri, police commissioner-general, said in a statement on Wednesday that police were investigating at least 100 cases of electoral fraud from the elections.
Several officials from the Zimbabwe Electoral Commission (ZEC) have been arrested after Mugabe's party claimed electoral officers had been bribed to count votes in favour of the opposition.
"This is a new phenomenon in the electoral history of Zimbabwe. We will leave no stone unturned in our quest to expose the source of this cancerous treachery. All those involved will be brought to book and prosecuted," he said.'Lucky to survive'
Jonothan Marikita, who was a parliamentary candidate for the MDC in the elections, told Al Jazeera on Tuesday that he was attacked with axes.
"They left me unconscious, I couldn't even talk. I was just lucky to survive, even now I don't know how I came here," he said from his hospital bed in Harare, where he was recovering from injuries sustained in an axe attack.