The independent Kenya National Commission on Human Rights group in a report released on Wednesday said that most of the victims "were felled by the bullet" during the recent turmoil across the East African country.
It added that authorities must account for "how the officers under their command used the live ammunition that had been assigned to them."
The group also reported over seven dozen cases of sexual or gender-based violence, saying it was "extremely worried" by the emergence of sexual violence "as a weapon of subjugation" during political contests.
The new allegations will add to pressure on the government to probe allegations of violence targeting opposition supporters.
A report by Human Rights Watch earlier this month said that Kenyan security forces raped, beat and assaulted civilians during the recent election turmoil. The report had also cited multiple gang-rapes by men in uniform in the slums of the capital Nairobi and the opposition strongholds of Kisumu and Bungoma.
"Some were raped in the presence of family members, including young children," the report said. "Most women said they were raped by policemen or men in uniform, many of whom carried guns, batons, tear gas canisters, whips, and wore helmets and other anti-riot gear."
The Kenyan police have rejected the allegations in the report as "utter falsehoods" and challenged Human Rights Watch to produce evidence.
Kenya has been in turmoil since September when the Supreme Court nullified the re-election of President Uhuru Kenyatta over "irregularities and illegalities."
The court ordered a rerun in October that was boycotted by opposition leader Raila Odinga, handing Kenyatta a landslide of 98 percent of votes cast by just 39 percent of the electorate.
Odinga boycotted the court-ordered repeat vote, saying electoral reforms had not been made. He has accused Kenyan security forces of killing scores of his supporters.
Dozens of people have been killed during violent clashes since the August 8 vote, which split the African country along ethnic and regional lines.