"Ethiopia's government has systematically assaulted the country's independent voices, treating the media as a threat," said Leslie Lefkow, deputy Africa director at Human Rights Watch.
State-owned radio, television and newspapers dominate Ethiopia's media landscape while independent journalists face threats, intimidation and harassment, said the group.
"The ruling party has treated the private media as a threat to its hegemony, and is using various techniques to decimate private media, independent reporting, and critical analysis, with drastic results," said the report, titled "Journalism Is Not a Crime".
Since the last polls in 2010 at least 60 Ethiopian journalists have fled into exile and 19 have been locked up, the report said.
"Muzzling independent voices through trumped-up criminal charges and harassment is making Ethiopia one of the world's biggest jailers of journalists," Lefkow said.
Ethiopia routinely dismisses such reports, and has previously said those arrested were not detained for their work as journalists but for "serious criminal activities".
International journalists and media houses are not spared. Ethiopia jailed two Swedish journalists in 2011 on terrorism charges and jams radio signals and blocks websites of those deemed too critical.
Internet and social media are also heavily restricted. The government has a telecoms monopoly and in 2012 banned the use of Skype.
In April a group of bloggers known as Zone 9, who write on social issues in the local Amharic language, were jailed awaiting trial on charges of terrorism and "outrages against the constitution".
Ethiopia's ruling EPRDF (Ethiopian People's Revolutionary Democratic Front), in power since 1991, occupies all but one of parliament's 547 seats and is widely expected to claim a landslide victory in May's polls.
The party adheres to a Chinese-style of authoritarian control aimed at driving economic growth of around 8.5 per cent, according to the International Monetary Fund.