"CIVICUS calls on the Kenyan authorities, politicians and leaders to act in a responsible manner and respect the will of the electorate during and after the elections", said the group.
Many people are expected to vote along ethnic lines.
The candidates are from opposing tribes, and current President Uhuru Kenyatta has been accused of rigging the elections process by his rival Raila Odinga, who also participated in the 2007 presidential elections and refused to accept his defeat, which led to the bloody clashes between tribes. "I am willing myself to accept the will of the people, so let them too", he said as he voted at the Mutomo Primary School in Gatundu, his home turf. He pulled an impressive crowd during the opposition National Super Alliance's (Nasa) final rally in Nairobi on Saturday, but still, many of his supporters say they believed President Uhuru Kenyatta would win again.
In 2013, Raila Odinga turned to the courts claiming electoral fraud, and lost.
A decade ago, vote tallying was abruptly stopped and the incumbent president declared the victor, triggering an outcry from Odinga's camp followed by outbreaks of ethnic violence in which 1,200 people were killed and 600,000 displaced.
The election day is upon Kenyans and many are heading to polling stations to cast their votes in this highly anticipated Tuesday, August 8 election. First, the vice president's rural home was attacked by a machete-wielding intruder and a little more than a week before elections, the man in charge of Kenya's electronic voting system was murdered.
Former President Barack Obama, who has largely stayed silent since President Donald Trump took over in January, opted to speak out about the hotly contested presidential election scheduled for Tuesday in Kenya, where voting in recent years has been followed by violence.
"In the unlikely event that I lose I don't need a speech, I will just speak from my heart", Odinga said shortly before voting.
Although there are eight presidential candidates on the ballot paper, the real battle is between incumbent Kenyatta, the 55-year-old son of Kenya's founding president and his longtime rival, 72-year-old Odinga, son of the country's first vice-president.
Tokoyian Ole Siamon, a mother of six decked out in traditional beads while waiting to vote, said she doesn't want any fighting if Kenya's vote is disputed.
"We will ensure that the long wait in queues is limited", electorial commission chairman Wafula Chebukati said on Monday evening.
Three polling stations in the Laikipia area were "affected by insecurity", but security forces restored order and polling resumed, Kenya's election commission said.
Kenya's general elections kicked off on Tuesday, with almost 20 million eligible voters expected to cast their ballots at over 40,000 polling centers to elect public officials, including the president.
The two dynasties have always been rivals, and Kenyans feel so strongly about the vote that ambulances have been ferrying bed-bound patients to polling stations. They expect changing demographics and new technology to help keep the vote fair and peaceful.
Former South African President Thabo Mbeki is one of the election's observers.