A video shows a man in a striped shirt and brown trouser attacking Martha Miano, an observer for the Political Parties Liaison Committee.
Trouble started after a supporter asked an official about IEBC badges.
He said: "This is very peaceful ward ... how are you getting campaigners and giving them badges for IEBC officials [yet they do not work there]?"
The official tells the MP that he explained the badge acquisition process but a relentless Gachagua says the commission's officials are being unfair.
"You gave her a badge and she is observing on your behalf," he says without identifying the woman he is referring to.
Miano is slapped seconds later.
"Unaongea nini, wewe (Why are you speaking)? the attacker asks her.
The observer tries to escape but the man chases after her while shouting.
He says: "Unatusi mweshimiwa (You are abusing the MP) ... who do you think you are? Anatusi mweshimiwa (She is abusing the MP)... ni wa rika yako (Is he your age mate)?"
A crowds that includes a policeman watches as events unfold.
Miano is rescued by a policewoman who whisks her into a police station to prevent the matter from escalating.
The observer had reportedly gone to report election malpractices at the station when the chaos ensued.
Kenyans on Twitter said the actions of the man were unacceptable so he should be punished.
"How can a man do this to a woman? He should be arrested and prosecuted, despite the cause, to set an example," one Patrick Igane said.
Dan Wanyama said the government should ensure the attacker faces dire consequences.
"Everybody should be respected regardless of status," he said, while Ba Fanuel noted that he matter was handled wrongly.
Via Twitter yesterday, the ODPP said investigations were on and that directions would be issued.
Courtesy Nancy Agutu