Video from a demonstration Monday at the offices of the Independent Electoral and Boundaries Commission in Nairobi shows riots officers firing tear gas and beating protesters with batons, including a man who was repeatedly struck and kicked after being taken down by police.
Kenya's police chief has called for an internal investigation, according to Interior Ministry spokesman Mwenda Njoka.
Macharia Njeru, chairman of the Independent Policing Oversight Authority, told reporters that a segment of the police force were notorious for "excessive force" and acted outside of the law.
Video showed largely peaceful demonstrators chanting, blowing whistles and carrying signs. In another part of the video, men in the crowd are seen breaking slabs of stone into smaller pieces. There were reports of rocks being hurled at officers but that was not captured on video.
Later, the video shows plumes of white smoke rising above the crowds. Some officers fired tear gas while other flailed their batons at protestors who appeared to be trying to get away.
In a scene widely circulated on social media, a man in a green hooded sweatshirt is chased and taken down by riot officers, who strike him with batons and kick him multiple times as he lay motionless on the ground. The beating has sparked public outrage and the hashtag #StopPoliceBrutality.
The man, identified as Benard Ngari Ngatia, later told Capital FM radio that he was applying for a job when he was caught up in the police response.
"The police started to beat me," he told the station. "People thought I had died. Even the police thought I was dead."
Ngatia said he was going to a hospital for a checkup and that he would sue the police if he could afford it.
The weekly protests have been organized by the opposition Coalition for Reform and Democracy, led by former Prime Minister Raila Odinga, who lost the 2013 election and unsuccessfully challenged the result in court.
The opposition accuses members of the Independent Electoral and Boundaries Commission of bias and has requested that the commissioners be dismissed. Commission members deny the accusations.
The U.S. Embassy in Nairobi condemned the police actions, as did Amnesty International.
"The brutal beatings by police ... amount to arbitrary and abusive use of force, which is illegal under Kenyan, regional and international law," said Muthoni Wanyeki, Amnesty International's Regional Director for East Africa, the Horn and the Great Lakes.
"The Independent Police Oversight Authority must quickly launch a thorough investigation into this blatant violation of human rights. Police officers suspected of responsibility for arbitrary or abusive use of force, including those with command responsibility, must be prosecuted in fair proceedings," Wanyeki said in a statement.
In 2013, the Kenyan Supreme Court upheld Uhuru Kenyatta's victory in the presidential election, dismissing Odinga's challenge.
Kenyatta and Odinga are the sons of the nation's first president and vice president, respectively, bringing back memories of a political dynasty that dates to the 1960s.
Their fathers started out as allies in the quest for Kenyan independence from Britain.
But the elder politicians' relationship ended in bad blood when founding President Jomo Kenyatta forced out his vice president, Jaramogi Odinga, following a series of disputes.
Kenya is East Africa's biggest economy and a crucial trade route into the rest of the continent.
It provides an important buffer of stability in a region that includes the fledgling Somali government and the politically tense Sudan and South Sudan.
Kenya is also a major U.S. ally in the war against Islamist militants in the region and has remained relatively peaceful amid civil wars in neighboring nations.