Voters in Ugenya and Embakasi constituencies began casting their ballots on Friday in hotly contested seats.
The two parliamentary seats became vacant following nullification by court of the election of Julius Mwathe and Christopher Karan.
In Ugenya, voting kicked off at 6 am as scheduled and long queues were formed at various polling stations.
There were few hitches at some polling stations after the Electronic Voter Identification kits failed to read fingerprints of voters.
ODM’s candidate Mr Karan cast his ballot at 8am at Bar Oninge Primary School polling station in West Ugenya.
Speaking to journalists after voting, Mr Karan claimed some of his supporters were attacked on the eve of the election and one person was hacked.
He said the attack happened after two rival groups clashed.
"He was taken to Sega Mission Hospital and is in stable condition," said Mr Karan.
Mr Karan said one of the vehicles used by the suspects was towed to Ukwala Police Station.
Ugenya police boss Willy Simba confirmed there was violence Thursday night and said three people were arrested in connection to the incident.
The campaigns in Ugenya have been marred by violence leading to the arrest and charging of at least six people.
The constituency has 57, 603 registered voters and 112 polling stations across the four wards, East Ugenya, West Ugenya, North Ugenya and Ukwala.
The constituency recorded a voter turnout of 81.9 per cent in the 2017 parliamentary seat election.
In Embakasi South polls opened at 6am.
The parliamentary seat that fell vacant in December last year after a successful petition.
Former MP Irshad Sumra successfully filed a petition challenging the election of Mr Julius Mawathe culminating in the Supreme Court’s decision of December 21, 2018 nullifying the election.
IEBC chairman Wafula Chebukati, while inspecting the voting process in some polling stations, said he expects that the exercise will be smooth all the way.
“I wish to call on the residents to come out and vote. Not just here in Embakasi South but also in Ugenya where another by-election is going on,” said Mr Chebukati.
According to some voters who spoke to Nation.co.ke, voting was fast enough despite the minor hitches of voters not knowing where their names fall in the polling booths.
At the tallying centre (Chaminade Training Centre) not much was going on Friday morning.
Polls will be closed at 6pm.
In Elgeyo-Marakwet, low voter turn out has marked the by-election in Lelan Ward.
IEBC presiding officer at Kamasat polling station Kiptoo Musa said that by 9.45am only 35 voters out of the 417 registered had cast their vote.
"We hope that everything will be okay at the end of the day. We have low voter turn out because most people were not ready, but we can not rule out the possibility of voters coming in large numbers in the afternoon," he said.
Mr Musa said that the process was going on smoothly so far.
Five candidates are battling out to win the seat that fell vacant after the death of Vincent Tanui last year.
The candidates are: Priscilla Kurgat, of Jubilee, Timothy Kemboi (Maendeleo Chap Chap), Pius Kwambai (Chama Cha Mashinani), Philip Chelimo (Grand Dream Development Party), and Salome Biwott (Kadu Asili).