"According to different independent sources, more than 300 persons were killed since the elections on December 27, 2007," the Kenya Human Rights Commission (KHRC) and the International Federation for Human Rights (FIDH) said.
The post-election chaos in Kenya worsened on Tuesday when a violent mob killed 30 frightened villagers by torching the church they were sheltering in.
According to residents and a security source, around 200 people had fled to the Kenya Assemblies of God Pentecostal church near Eldoret town in an attempt to escape vigilante gangs roaming the streets.
Deadly tribal clashes have raged across parts of Kenya since the disputed re-election of President Mwai Kibaki on Sunday.
According to one local journalist who saw the burned corpses, "Some youths came to the church. They fought with the boys who were guarding it, but they were overpowered and the youths set fire to the church.”
The victims, which witnesses said included women and children, were largely Kikuyus from the same ethnic group as Kibaki. The area is traditionally dominated by the Kalenjin tribe and when ethnic violence broke out in 1992 and 1997, hundreds were killed and thousands displaced.
A Catholic priest in Eldoret, Fr. Paul Brennan, said it was too dangerous for people to go outside.
"There are four to five thousand [sheltering] in the main cathedral, and thousands in other churches," he said. "Houses are being burned. It is too dangerous to go outside and count the dead."
Police spokesman Eric Kiraithe spoke of his shock over the level of the violence in a news conference in the Kenyan capital Nairobi.
"This is the first time in history that any group has attacked a church. We never expected the savagery to go so far,” he told journalists.
Kiraithe said that reinforcements were beings rushed to the Eldoret area to arrest perpetrators of violence “regardless of their status in society.”
British Prime Minister Gordon Brown pressed the African Union and the Commonwealth on Tuesday to intervene in the chaos by urging Kibaki and defeated opposition leader Raila Odinga to talk.
"What I want to see is them coming together; I want to see talks and I want to see reconciliation and unity," Brown said. "I want to see the possibility explored where they can come together in government.
"But the first priority is that the violence is brought to an end. It is unacceptable that lives are being lost," he said.