Armed gangs are seizing children for ransom payments in the lawless north of the Central African Republic
Some parents have paid up to $4,000 for their children to be freed - other minors have been killed, Amnesty says.
The region is a "free-for-all" for rebels, soldiers and armed bandits, a researcher who has just returned says.
CAR has accused neighbouring Sudan of backing the rebels, with attacks coming from Darfur. Sudan denies the charges.
Some families have had their children kidnapped seven times, says Amnesty researcher Godfrey Byaruhanga.
"News is clearly spreading to criminal elements throughout the region that they can have free rein in northern CAR, as there is an almost total absence of any authority," he said.
Civilians were fleeing "from the frying pan into the fire" by heading to Chad and Sudan, he said.
More than 280,000 people have fled their homes in the past year.
Rebels killed anyone who refused to fight with them, while government soldiers killed suspected rebel sympathisers, he said.
Presidential spokesman Cyriaque Gonda denied that the army was killing civilians.
"This is not true, this is a ridiculous accusation and those accusations not founded at all," he told the BBC's Focus on Africa programme.
"What the government troops are doing is first of all to protect themselves when they are being attacked and to go after the rebels who are keeping the population under terror."
Mr Byaruhanga said the government was failing to protect the population and only controlled the capital, Bangui.
Mr Gonda renewed a call for the UN to send peacekeepers - an idea also supported by Mr Byaruhanga.
"This situation is too dangerous and simply cannot wait," he said.
Desperate Villagers Flee Central African Republic