The electoral commission has warned Kibra voters against sharing on social media their marked ballot papers.
In a tweet, the Independent Electoral and Boundaries Commission (IEBC) said violating secrecy of the ballot is punishable by law.
"Voters are warned that violating secrecy of the ballot through sharing of photos of marked ballot papers is punishable in law #byelections2019 #talkwithIEBC," said the IEBC on Thursday.
The IEBC also informed voter assistants, those who escort either the disabled or elderly, that they can only assist one person in the election and must sign the oath of secrecy.
Further, the electoral agency assured voters that the voter identification gadgets are functioning "optimally" and urged the over 100,000 voters to turn out to exercise their democratic right.
The Elections Offences Act Section 3A prohibits any person from being in possession of any ballot paper which has been marked with any official perforation, stamp and or mark, but the law does not specify if the possession has to be physical or digital.
The Act also prohibits voters from taking out of the polling station any ballot papers.
The offences are punishable by a five-year jail term.
Voting in the Kibra by-election kicked off at 6 am and several of the participating candidates have cast their ballots.