Security forces have opened fire on demonstrations in the past
The Yemeni government has created a "climate of fear" in the south of the country and has unlawfully killed civilians, campaigners say.
New York-based Human Rights Watch has released a report accusing the government of human rights abuses against a secessionist movement there.
Yemen used to be two separate countries which were united in 1990 under President Ali Abdullah Saleh.
The south of the country says it is economically exploited by the north.
Human Rights Watch say that government forces have used lethal force against demonstrators during protest marches.
The latest in a series of incidents was reported in November when three protesters and two soldiers were killed at a secessionist rally in Ataq, Shabwa province.
"They surrounded us from every side and started shooting," Nasser Huwaider, a member of the secessionist Southern Movement, told the Associated Press news agency.
Many protesters were later arrested, he said.
The Human Rights Watch report says the government security forces have also led a crackdown on freedom of speech and assembly.
The authorities have suspended publication of several newspapers, censored coverage of sensitive issues and arrested popular bloggers, Human Rights Watch say.
In 1994 there was a civil war in Yemen after an attempt by the south to secede was quashed by the northern government of Mr Saleh.
People in the south, home to most of Yemen's oil facilities, have long complained the central government takes advantage of their resources but marginalises and discriminates against them.
The government's battle against Houthi rebels in the north of the country has dominated headlines since fighting intensified in August.
But Human Rights Watch say that attention also needs to be brought to the government's actions in the south.
Yemen's president says the government is trying to hold the country together and prevent it descending into a failed state.
"Yemenis have to learn a lesson from what had happened in Iraq and Somalia," Mr Saleh said in May this year.
Saudi Arabia and the United States have both warned Yemen could become a potential hide-out for al-Qaeda militants.