The rights commission said more than 20 people died in violence in CAR last weekend. Commissioner Navi Pillay said she was alarmed the conflict.
"The situation on the ground is extremely worrying," she said in a statement.
The rebel Seleka coalition seized control of the government in CAR. Ousted President Francois Bozize fled to Cameroon and rebels suspended the constitution.
U.N. Secretary-General Ban Ki-moon said there were emerging reports of rape, the use of child soldiers and other grave violations of international laws in CAR.
Pillay said there has been a wide-range of abuses committed since Seleka launched its offensive in December. At least 37,000 people have fled the country due to the violence.
"Those carrying out serious crimes, and especially their leaders, should bear in mind that they may be held individually criminally responsible," she said.
Bozize told the BBC that CAR forces had "neutralized" Seleka fighters, who were later supported by "Chadian special forces" in the takeover.
Bozize said he resigned to avoid more bloodshed.