Kenyan officials have been asked to rein in police officers accused of using excessive force during the election period.
"Kenya has to put an end to the culture of election-related violence, unlawful killings by police and impunity for abusive officers," said Otsieno Namwaya Africa researcher at Human Rights Watch.
Mr Namwaya also called on the authorities to investigate scores of deaths that occurred during mass protests in the recent past.
"President Uhuru Kenyatta and other government officials should condemn ongoing killings and ensure that there are thorough and independent investigations into such killings and the role of any armed groups in the violence," he said.
In October, Human Rights Watch and Amnesty International documented at least 67 killings, most by gunshot, during protests by opposition supporters after the electoral commission declared Kenyatta the winner of the August 8 election.
The Human Rights Watch indicated that a series of protests and clashes between police and opposition supporters began on November 17, 2017, at the Nairobi airport where supporters of the opposition leader Raila Odinga escorted him to the town center.
They pointed out that media reports said unidentified gunmen shot at Mr Odinga's car and either shot or beat to death dozens of people.
"The protests and clashes continued in opposition strongholds in Nairobi and western Kenya following the Supreme Court decision on November 20 affirming President Uhuru Kenyatta's re-election," said their statement.
The rights group pointed out that the latest round of violence started after the authorities deployed police who used teargas and water cannons to break up large crowds cheering Mr Odinga.
"A photojournalist told Human Rights Watch that he witnessed four apparently unarmed supporters in Odinga's convoy shot by police along Landhies road and Haile Selassie Avenue, as the convoy entered central Nairobi," added the report.
This comes as the police spokesman, George Kinoti, said that the crowds had beaten five people to death on November 17 for looting before the police arrived and promised to investigate the deaths.
According to reports by local media and local human rights activists, said Mr Namwaya, more than 10 people are believed to have been killed between November 18 and November 20 while international media have reported at least 24 people killed by police since November 17.
On the other hand, human rights activists and a community mobiliser told Human Rights Watch that, since November 17, they had witnessed police killing protesters in Nairobi's Dandora Phase Four, Kibera, Mathare and Kawangware neighbourhoods, as well as in Kisumu and Migori in western Kenya.
On November 19, opposition supporters engaged police in running battles in many parts of Nairobi as they protested the killing of another five people by unidentified attackers.
On the morning of November 19, the bodies of four men and one woman, believed to be opposition supporters, were found along the Nairobi-Thika highway near their homes in Riverside neighbourhood, Ruaraka area, residents and local leaders said.
Neighbours who witnessed one of the attacks told Human Rights Watch they believed the attackers were members of Mungiki, a pro-government armed group responsible for many killings in the 2007-2008 post-election violence.
But Nairobi police boss, Japheth Koome, dismissed the allegations and promised to launch investigations.
Mr Namwaya however noted that the authorities have failed to investigate similar attacks or to hold anyone responsible for the killings by police since the August 8 election.