A screengrab from the viral video.

Police in Kisii have arrested a man suspected to have assaulted a woman during a Kenya Kwanza rally in Kisii on Thursday after a viral video.

Police said their attention was drawn to a disturbing and widely circulated video clip where a man is clearly seen assaulting a woman during the rally.

"Police in Kisii have acted on the incident and made an arrest of one male person .... He is in police custody assisting with the investigation and will face court action once the investigation is completed," read a tweet by the National Police Service.

The statement said, "National Police Service wishes to advise the public to exercise restraint and accommodate divergence of opinion during the electioneering period to mitigate on instances of lawlessness and breach of the peace."

The incident happened at Ibacho grounds in Nyaribari Masaba Constituency where Deputy President William Ruto, ANC party leader Musalia Mudavadi and Ford-K's Moses Wetang'ula were addressing a political rally.

The woman was among a group chanting the Azimio la Umoja slogans as the DP addressed the crowd.

In the video, the suspect approaches the woman from behind, stretches his arm and slaps her before greeting those standing next to him.

A contingent of police officers in anti-riot gear had been deployed to the venue as early as 6 am. The leaders arrived for their address a few minutes past 2pm.