UN chief Ban Ki-moon says the Central African Republic (CAR) has suffered a "total breakdown of law and order" since rebels seized power in March.
He urged the UN Security Council to consider sanctions or to set up a panel of experts to monitor the situation.
Seleka rebel group leader Michel Djotodia ousted President Francois Bozize in March.
Last month Medecins Sans Frontieres (MSF) said the country's health care system had collapsed.
In a report that the Security Council is due to discuss on Wednesday, Mr Ban said infighting among rebel groups had led to widespread abuses.
"They included arbitrary arrests and detention, sexual violence against women and children, torture, rape, targeted killings, recruitment of child soldiers and attacks, committed by uncontrolled Seleka elements and unidentified armed groups throughout the country," the report said.
He said the situation was particularly chaotic in rural areas where Seleka forces "continued to wreak havoc".
Mr Ban said that some 1.6 million people were now in urgent need of assistance including protection, food, water, health care and shelter.
"This is unacceptable. The plight of the people of the CAR must be brought to an end," he said.
"I call on the Security Council to consider appropriate options, including the adoption of sanctions or the establishment of a panel of experts, to ensure there is no impunity for perpetrators of gross human rights violations."
Last month, MSF said that it - along with the UN and other international agencies - had been a victim of robberies and looting in recent months.
In a report it said malnutrition and preventable diseases were rife, with 33% more malaria cases reported this year then in the same period last year.
After seizing power, Mr Djotodia pledged to hold elections after an 18-month transitional period.
In April, regional states agreed to send 2,000 peacekeepers to bolster a 500-strong multinational force that was battling to help the interim government restore stability.
CAR has an unstable history and is extremely poor, though it has large deposits of minerals including gold and diamonds.