A 10-year old girl was shot dead by police firing "sporadic shots" at protesters in Mathare Area 2 on Saturday morning.
The class four pupil was killed while playing at the balcony of her parents' home.
The neighbourhood is loyal to Opposition leader Raila Odinga, whose party has rejected the results of Tuesday's vote as a "charade".
Kenyan police killed at least 11 people in a crackdown on protests as anger at the re-election of President Uhuru Kenyatta erupted in Kisumu and slums surrounding the capital, officials and witnesses said on Saturday.
The bodies of nine young men shot dead overnight in Nairobi's Mathare slum were brought to the city morgue, a security official told Reuters, adding that the men had been killed during a police anti-looting operation.
Another government official told reporters another man was killed in Kisumu county.
Kisumu's main hospital was treating four people brought in overnight with gun-shot wounds and six who had been beaten by police, hospital records showed.
One man, 28-year-old Moses Oduor, was inside his home in the impoverished district of Obunga when police barged in after midnight as part of house-to-house raids, dragging him out of his bedroom and laying into him with clubs.
"He was not out fighting them, he was rescued by my sister who lives next to him, she came outside screaming at the police asking why they are beating people," his brother, Charles Ochieng said, speaking on behalf of a dazed Oduor.
More shooting was heard outside the hospital on Saturday morning. In Nairobi, local television showed footage of armed police units backed by water cannon moving through the rubble-strewn streets of Kibera, another pro-Raila Nairobi slum.
IEBC announced its official results late on Friday, giving 55-year-old Uhuru another five years in power after securing 54.3 per cent of votes cast.
Raila's NASA coalition rejected the results even before they were announced, saying that the election commission's systems had been hacked, that the conduct of the count was irregular and that foreign observers who gave the poll a clean bill of health were biased.
Acting Interior CS Fred Matiang'i has denied that police are using live bullets against protesters.
He also said he is unaware of any deaths as a result of confrontations with security forces.
In Kibera, a convoy of pick-up trucks driving senior NASA officials into the slum was turned back by volleys of live police fire and tear gas, a Reuters witness said. It was not clear why the officials were going to Kibera.
Raila loyalists fleeing from security forces in Kibera vowed to vent their rage at the seat of Kenyatta's administration in central Nairobi.
"This is just a warm-up. Tomorrow we will go to State House and they can kill us there," shouted Felix Oduor, 18, as he ran from clouds of tear gas along Kibera's railway line. "They can't kill us all," those around him shouted in response.
"THEY ARE CRIMINALS"
Matiang'i denied accusations of brutality, saying police were merely maintaining order.
"Let us be honest - there are no demonstrations happening," he told reporters.
"Individuals or gangs that are looting shops, that want to endanger lives, that are breaking into people's businesses – those are not demonstrators. They are criminals, and we expect police to deal with criminals how criminals should be dealt with."
Even before the declaration, NASA coalition had rejected the outcome, saying the election commission's systems had been hacked, the count was irregular and foreign observers who endorsed the poll and the count were biased.
NASA provided no evidence for any of its accusations but singled out former US secretary of state John Kerry and former South African president Thabo Mbeki - who both led teams of election observers - for criticism.
Top lieutenant James Orengo said NASA would not challenge the results in court - as Odinga did when he lost in 2013 - but hinted at mass action by praising the history of Kenyans in standing up to previous "stolen" elections.
"Going to court is not an option. We have been there before," Orengo told reporters.
In addition to the thumbs-up from foreign monitors, Kenya's ELOG domestic observation group, which had 8,300 agents on the ground, published a parallel vote tally on Saturday that conformed with the official results.
ELOG's projected outcome put Kenyatta on 54 percent with a 1.9 percent error margin - compared to an official tally of 54.3 per cent.
"We did not find anything deliberately manipulated," ELOG chairwoman Regina Opondo said.