In its most comprehensive report into the Oromo protests, HRW lists the names of more than 300 it says were killed.
The government has acknowledged that protesters have died but said HRW was “very generous with numbers”.
Protests were sparked by fears that a plan to expand the capital into Oromia region would displace Oromo farmers.
They began in November last year, but the government dropped the proposal to enlarge Addis Ababa in January.
Oromia is Ethiopia’s largest region, completely surrounding the city.
The change of policy has not stopped the demonstrations, but they have reduced in their intensity.
At the last census in 2007, the Oromo made up Ethiopia’s biggest ethnic group, at about 25 million people out of a population at the time of nearly 74 million.
An investigation, released last week, by the Ethiopia Human Rights Commission, appointed by parliament, found that 173 people had died during the unrest.
It said the dead included 28 security officers and local government officials.
Information Minister Getachew Reda said that in the main the security forces conducted themselves “in a very professional and responsible manner”.
He put the killings down to “a few bad apples”.
The government has said that it will investigate and deal with those responsible.
Some of those interviewed allege they were hung by their ankles and beaten while others described having electric shocks applied to their feet while in detention.
Several women also claim to have been raped and sexually assaulted.
HRW also says that tens of thousands of people were arrested and hundreds have disappeared