Mark Lipdo of the Stefanos Foundation said he witnessed the massacre
Hundreds of people, including many women and children, were killed in ethnic violence near the city of Jos in Nigeria at the weekend, officials say.
They said villages had been attacked by men with machetes who came from nearby hills.
Troops have now been deployed in the area and dozens of arrests are said to have been made.
Acting President Goodluck Jonathan has ordered security forces to prevent more weapons being brought into the area.
He has also sacked the country's national security adviser, Sarki Mukhtar.
Jos has been under a military curfew since January when at least 200 people died in clashes between Christians and Muslims.
Caroline Duffield, BBC News, Lagos
Already this is being described as retaliation for the outburst of killing in January in which hundreds more people were killed.
Back then the largest losses were suffered by the Hausa Fulani community. In the village of Kuru Karama more than 100 people were killed and their bodies thrown into wells and sewers. Grave accusations were made that the local government had stoked the violence. This time it is clear that the targets were Berom Christians.
For weeks there have been rumours of retaliation in these villages and people have been living in a state of anxiety. Many families left. These killings are often painted by local politicians as a religious or sectarian conflict. In fact it is a struggle between ethnic groups for fertile land and resources in the region known as Nigeria's Middle Belt.
The latest attacks, on members of the mainly Christian Berom community, are said to have been reprisals for the January killings.
The authorities say the villages are now calm after troops and military vehicles entered them.
An adviser to the Christian-dominated Plateau state government, Dan Manjang, told AFP: "We have been able to make 95 arrests but at the same time over 500 people have been killed in this heinous act."
Another Plateau state official, Gregory Yenlong, urged people to "remain calm and be patient as the government steps up security to protect lives and property in this state".
Many of the dead in the villages of Zot and Dogo-Nahawa are reported to be women and children.
Chief Gabriel Gyang Bot, from nearby Shen, told the BBC's Focus on Africa programme that people in his village feared more attacks.
He said he had received text messages from people who claimed responsibility for the weekend attacks and had threatened to return.
Mark Lipdo, from the Christian charity Stefanos Foundation, said Zot village had been almost wiped out.
He said: "We saw mainly those who are helpless, like small children and then the older men, who cannot run, these were the ones that were slaughtered."
A resident of Dogo-Nahawa said that the attackers had fired guns as they entered the village before dawn on Sunday in defiance of a curfew.
JOS, PLATEAU STATE
Deadly riots in 2001, 2008 and 2010
City divided into Christian and Muslim areas
Divisions accentuated by system of classifying people as indigenes and settlers
Hausa-speaking Muslims living in Jos for decades are still classified as settlers
Settlers find it difficult to stand for election
Communities divided along party lines: Christians mostly back the ruling PDP; Muslims generally supporting the opposition ANPP
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