Hailing the electoral commission as "heroes" after he was declared the winner on Monday, Ruto said: "There is no looking back. We are looking to the future. We need all hands on deck to move forward."
The 55-year-old had made Kenya's class divisions the centrepiece of his campaign to become Kenya's fifth president, promising to reward low-income "hustlers."
He was also scornful of Kenya's political dynasties - his opponent Raila Odinga and President Uhuru Kenyatta, son of the nation's first vice president and president, respectively.Â
Kenyatta, who has served his two-term limit as president, fell out with Ruto after the last election and this time endorsed Odinga, making his fifth attempt at winning the presidency.
Once in office, Ruto will have to confront an economic and social crisis in East Africa's most advanced economy, where poor Kenyans already reeling from the impact of COVID-19 have been hit by global rises in food and fuel prices.
The worst drought for 40 years has devastated the country's north, leaving 4.1 million people dependent on food aid, while its debt levels have soared.
Ruto, who heads the Kenya Kwanza (Kenya First) Alliance, had appeared to be leading opposition leader Odinga as Kenyans awaited final results of the election held nearly a week ago.
Minutes before the chairman of the electoral commission Wafula Chebukati announced that Ruto had won, his deputy Juliana Cherera had told media at a separate location that she and three other commissioners disowned the results.
"We are not able to take ownership of the results that will be announced, because of the opaque nature of this last phase of the general election," she said.
The electoral commission has seven commissioners.
Before announcing Ruto as the winner, Chebukati said two commissioners and the electoral commission's chief executive had been injured and were being treated.
Chebukati said Ruto had won 50.49 per cent of the vote, against Odinga's 48.5 per cent.
The winning candidate must get 50 per cent of votes plus one.
The United Nations took note of the results, UN spokesman Stephane Dujarric said, and encouraged all candidates to "resort to legal channels to address any challenges that may arise."
The US embassy in Kenya urged all parties to work together to peacefully resolve concerns about the election.
"We ask all political party leaders to continue to urge their supporters to remain peaceful and refrain from violence," it said in a statement.
Odinga did not attend the announcement. His running mate, Martha Karua later tweeted: "It is not over until it is over."
Several plumes of black smoke rose in the streets of Kibera, mirroring similar scenes in Kisumu, to the west of the country, as people burned piles of tyres. Police fired tear gas to disperse the crowds.
In Kisumu, Governor Anyang' Nyong'o called for calm as protests in parts of the lakeside town turned violent after Ruto's victory was declared. He said the Azimio leadership was working to plan its response to "ensure justice" for Odinga.
Amid shouts of "We need Raila now!", "Chebukati must go!" and "No Raila, no peace!", motorcycle drivers honked their horns and people blew into vuvuzelas and whistles.
By contrast, the mood in Eldoret - Ruto's home turf - was ecstatic.
