Democratic Center party candidate Ivan Duque won the Colombian presidency in Sunday's runoff vote, election authorities said.
With 99.56 percent of the ballots counted, Duque garnered an unprecedented 10.3 million votes, or 53.95 percent, while his runoff rival, Gustavo Petro with the Colombia Humana movement, received about 8 million votes, or 41.83 percent.
Blank ballots have totaled 4.2 percent so far, or 795,510 votes, significantly below the 15 percent that some surveys had predicted would be cast.
Santos telephoned Duque to congratulate him on his victory, later writing on his Twitter account: "I called Ivan Duque to congratulate him and wish him the best of luck. I offered the government's full cooperation to ensure an orderly and calm transition."
During the campaign, Santos had not expressed his support for either of the two main candidates, who on Sunday he called "opponents" of his administration.
Santos after voting in Bogota called the process a "momentous election," highlighting the fact that it was held in peace.
"In the last election's first round, a former FARC commander voted democratically for the first time ever," Santos said, referring to guerrilla chief-turned-political party leader, Rodrigo Londoño.
Duque - who had remained ahead in the polls in the run-up to Sunday's vote - received the backing of former President Alvaro Uribe, who said after casting his ballot that the Democratic Center party leader will protect Colombia.
"I voted for them (Duque and running mate Marta Lucia Ramirez) because they guarantee socially inclusive growth," Uribe said. "They are a guarantee that Colombia will not fall into the destructive clutches of socialism."
More than 7.5 million Colombians voted for Duque in the May 27 first-round election, while Petro received 4.8 million votes, which translates to 39.14 percent and 25.08 percent, respectively.
The president-elect of Colombia, Ivan Duque, said Sunday that "a new generation" comes to power following his victory in the presidential elections, and stressed his intention to unite the country.
"With humility and with honor I want to tell the Colombian people that I will give all my energies to unite our country: no more divisions, let's think of a country with all and for all," Duque said in a speech to his supporters in Bogota after acknowledging the elections' results.
Duque added that for him, "there are no defeated citizens" because he wants to be the president of all Colombians: those who voted for him, those who did not and those who opted for a blank vote.
The president-elect of Colombia, who is due to take office on Aug. 2, 2018 and whose term would last until Aug. 2, 2022, further highlighted the need for Colombia to "unite" and "build" the country to prevent the future of all Colombians from getting "destroyed".