The rights group said it documented several incidents where human rights activists in Armenia were assaulted after taking part in September rallies.
"The police and other Armenian authorities need to be prepared to make clear that they are going to put a stop to these attacks, starting with effective investigations that lead to arrests and prosecutions," Giorgi Gogia, a South Caucasus research at Human Rights Watch, said in a statement from Berlin.
The rights group didn't indicate the reason for the demonstrations. Radio Free Europe/Radio Liberty said demonstrators rallied last week against plans announced by President Serzh Sarkisian to join the Customs Union of Belarus, Kazakhstan, and Russia.
Stefan Fule, neighborhood policy commissioner for the European Union, said Thursday he'd lead a delegation to Armenia this week for informal partnership talks.
"The EU remains firmly committed to moving this partnership forward as much as possible, based on the ambitions of our partners and their ability to implement necessary reforms," Fule said in a statement.
The EU's Eastern Partnership extends to Armenia, Azerbaijan, Belarus, Georgia, Moldova and Ukraine.