The Commonwealth Lawyers Association is calling on the Papua New Guinea government to take steps to protect its citizens and bring perpetrators of the deadly violence in three remote villages in the Momase region to justice.
Police said five out of the 30 men involved in the mass killing that occurred on 16 July have been arrested.
But the ring leaders, who are reportedly armed, are still on the run.
There are reports that almost 400 people have been displaced as a result and East Sepik Governor Allan Bird said the tribal violence has continued to deteriorate in the country.
The "barbaric killings" of 26 people, including women and children, in the Angoram District two weeks ago "are utterly unacceptable and a gross violation of human rights", the CLA said in a statement.
"The oldest and simplest justification for government is as protector: protecting citizens from violence," it said.
It said it agrees with the PNG Law Society that the rule of law must prevail to protect the people of PNG and their human rights.
The CLA is urging for "swift action by law enforcement agencies" and "calls for justice to be delivered through working collectively to ensure communities can live securely without fear of violence."
The group added that all necessary steps should be taken to ensure the rule of law is respected.
Survivors desperate
Meanwhile, about 200 survivors of the massacre have turned up at the Angoram Station desperate for help.
The United Nations is assisting with emergency relief, setting up six temporary care centres.
Five children have been identified as "unaccompanied" with their parents presumed dead or missing.
Deputy Commissioner of Police Regional Operations Samson Kua said they are continuing investigations and speaking with survivors to identify the criminal gang.
The Post-Courier reports Angoram MP Salio Waipo claims he knows the people involved, and has told them to turn themselves in.
RNZ Pacific correspondent in PNG, Scott Waide, said while additional police have been deployed, there was guarantee that villagers who witnessed the violence will be protected if they choose to testify in court.