Amnesty International said on Thursday it was "gravely concerned" by the outbreak of violence in Nepal's southern plains and urged Kathmandu to punish security forces if they are found guilty of rights violations.
At least 11 people have been killed and more than 100 injured over the last two weeks after ethnic Madhesi people living in the Himalayan nation's Terai lowlands launched protests demanding more places in a constituent assembly due to be elected later this year and in parliament.
Amnesty said many of the dead were believed to be victims of police shooting at protesters and urged authorities for "a prompt, independent, impartial and thorough investigation".
"Wherever there is sufficient evidence, prosecute anyone suspected of human rights abuses in proceedings that fully respect international fair trial standards," the group said.
"Amnesty International acknowledges the responsibility of the authorities to maintain law and order, but is concerned that some of the killings may be a result of the possible use of excessive force by police."
The Madhesi people complain of widespread discrimination, including under-representation in parliament, political parties, army and the police, despite being the majority in an area that is home to almost half of Nepal's 26 million people.
They are ethnically and culturally closer to people from the neighbouring Indian states of Bihar and Uttar Pradesh than to Nepalis from the mountains.
The violence has cast a shadow over a landmark peace pact between the government and Maoists ending a decade-old Maoist conflict that killed 13,000 people.
Amnesty said there had been reports of demonstrators attacking journalists and offices of newspapers or radio stations. Some journalists had fled the area to avoid being targeted, the group said.
It said there were also threats to human rights activists in the area and some teams were attacked during visits to assess the unrest.
On Wednesday, Prime Minister Girija Prasad Koirala vowed to turn Nepal into a federal state, ending central control and promised to increase the representation of ethnic groups including the Madhesis in the assembly meant to map out the impoverished country's political future.
There was no reaction from protest leaders but some activists said Koirala's pledge was not enough.