Sata was to be formally sworn in Friday by Zambia's chief justice, Voice of America reported.
Elections Director Priscilla Isaacs said Sata, who heads the Patriotic Front, received 1,150,045 votes in Tuesday's election to 961,796 for Banda, leader of the Movement for Multi-Party Democracy, and 489,944 for Hakainde Hichilema of the United Party for National Development.
"Mr. Michael Sata has already been declared president by the chief justice, who is also the returning officer for the presidential election, and the inauguration is going to take place today, because our law provides that, once the results are declared for a presidential election, the presidential should be sworn-in no later than 24 hours after the declaration," she said.
Isaacs said the electoral commission had done its job "very well."
"The preliminary results that we have received, some of the international observers have commended us for conducting a credible and transparent election," Isaacs said. "In terms of the results, we have given ourselves the 48-hour target in which to declare the election results, and we just about managed that target."
The polling was watched over by international monitoring agencies.
Isaacs said Zambians should have confidence in the electoral process, VOA said.
"We've conducted an election which has been credible and transparent," she said. "We can proudly, openly and honestly say that we have done a good job and conducted a good election."