"Security forces have killed at least 140 protesters and injured many more, according to activists, in what may be the biggest crisis to hit Ethiopia since the 2005 election violence," HRW's Felix Horne said.
The number reported by HRW is almost double the previous toll of 75 the group gave last month.
There was no immediate response from the Ethiopian government, which has previously put the death toll at five.
"Over the past eight weeks, Ethiopia's largest region, Oromia, has been hit by a wave of mass protests over the expansion of the municipal boundary of the capital, Addis Ababa," Horne said.
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"The generally peaceful protests were sparked by fears the expansion will displace ethnic Oromo farmers from their land, the latest in a long list of Oromo grievances against the government."
"By treating both opposition politicians and peaceful protesters with an iron fist, the government is closing off ways for Ethiopians to nonviolently express legitimate grievances," Horne said.
"This is a dangerous trajectory that could put Ethiopia's long-term stability at risk," he warned.
With at least 27 million people, Oromia is the most populous of the country's federal states and has its own language, Oromo, distinct from Ethiopia's official Amharic language.
Some 200 people were killed during post-election violence in 2005.