The Human Rights Watch group says Zimbabwe has failed to make key reforms needed to ensure free and fair presidential election later this year.
The group said this has provoked fears of a replay of the trouble surrounding the last poll in 2008.
It urged the government to move fast to implement legal and institutional reforms.
President Robert Mugabe, who will be 90 by election day, is expected to run for another term. He has sweeping powers under current laws.
The group said security forces, election bodies and state broadcasters are also politically partisan, remaining loyal to Mr Mugabe and Zanu-PF.
A BBC correspondent said the government has managed to halt hyperinflation and brought relative economic stability to the country over the last four years.