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, former president and civilian leader of the military junta now controlling the country.
It said the unrest was centred on areas that had traditionally been Zanu (PF) strongholds but in recent years had increasingly turned to the opposition.
The 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.