The United States condemned on Tuesday the "excessive use of force" by Kenya's security services during a demonstration by opponents of the electoral oversight body, the embassy said.
Police beat some demonstrators with batons and kicked others on Monday after firing teargas and water cannon to disperse a crowd outside the Independent Electoral and Boundaries Commission. Some demonstrators threw stones at police.
"The United States deplores the excessive use of force by the Kenyan security services and the violence around the demonstrations," U.S. Ambassador Robert Godec said. "We welcome announcements by Kenyan authorities that all reports of the excessive use of force will be investigated."
Kenya does not hold its next presidential and parliamentary polls until August 2017, but politicians are already trying to galvanise their supporters.
More than 1,200 people died after an election in 2007. A 2013 election passed peacefully, although the opposition disputed the result and said the commission mishandled the voting. It accepted a court ruling that rejected the challenge.
The commission oversees elections in Kenya, including ensuring voter lists are up to date and supervising counting. Kenya's opposition accuses the commission of lacking the impartiality to referee a fair vote, a charge its members deny.
Inspector General of Police Joseph Boinnet condemned what he called "lawlessness" by "rioters" but said an internal enquiry was under way to see if any officers broke the law, the Daily Nation reported on it website. Reuters could not immediately reach officers for comment.
"Police officers have been photographed attacking unarmed and peaceful citizens on the streets of Nairobi and Kisumu. This is utterly unacceptable," said the Law Society of Kenya, the professional body for the country's lawyers.
Protests also took place in Kisumu in western Kenya.
A Nairobi court charged 15 men, who participated in the protest, with creating a disturbance and being armed with stones, a court official told Reuters, adding that they had denied the charges.
Opposition leader Raila Odinga, who challenged the 2013 result and attended Monday's protest, is expected to stand again in 2017 against President Uhuru Kenyatta, who is expected to seek a second and final term.