The Central Election Commission (CEC) said turnout well exceeded the 33 percent minimum needed to make the vote valid.
Nizomiddin Nurmatov, the head of CEC's press service, told RFE/RL's Uzbek Service that as of 12:00 local time 59.8 percent of eligible voters had cast ballots, making the election valid.
Former Communist Boss
Karimov, a former communist party boss who turns 70 in January, is running against what analysts have described as nominal challengers.
The three other candidates who registered with the election commission are regarded as Karimov loyalists and have praised him publicly.
The legal basis for Karimov's candidacy remains unclear, since the Uzbek Constitution bars a president from serving more than two terms.
Karimov's second term expired at the beginning of 2007, and there was no explanation as to why authorities approved his bid last month.
Karimov has already extended his term in office twice through referendums, in 1995 and 2002.
In the run-up to today's election, Karimov told foreign diplomats that he wanted "a free society and prosperous life" for the republic of 27 million people.
But Human Rights Watch said on December 21 that none of the conditions exist for free and fair elections.
The New York-based group said Uzbekistan’s record of restricting opposition political activity and independent media prevents any semblance of a competitive electoral process.
Human rights activists inside Uzbekistan told RFE/RL's Uzbek Service that there was clear evidence of voting violations at polling stations. In the eastern city of Ferghana, rights defender Abdusalom Ergashev said, "I witnessed several voters casting ballots for their entire families." Ergashev added that election officials were coaching voters. "At every ballot-box a man and a woman are sitting and advising the voters on checking the proper box (candidate's name), that is voting for Islam Karimov,” Ergashev said
In the central city of Jizzakh, not far from Samarkand, rights defender Bahtiyor Hamroev said he went to several polling stations and " was appalled seeing that many voters a casting three, four or even six ballots into the boxes.”
A number of media organizations, including the BBC, have been refused permission into the country to cover the election. Viktoria Novikova, the Head of international department of the Russian Channel One, said her channel also did not receive permission to cover the election.
The Organization for Security and Cooperation in Europe (OSCE) has a small team observing the poll.
The OSCE said it would not conduct systematic and comprehensive observation of election-day proceedings due to the "apparent limited nature of the competition."
Molly Stephenson, the press attache at the U.S. Embassy in Tashkent, said, "We have a number of people out from the embassy, observing the election today and we're still collecting data right now."
Both the OSCE and the U.S. State Department are expected to release assessments of the Uzbek vote this week.
Western election monitors have never recognized an Uzbek parliamentary or presidential election as free and fair or as meeting international standards.
Polls were due to close at 20:00 local time. Preliminary results are expected early December 24. Exit polls are prohibited by law.
(RFE/RL's Uzbek Service contributed to this report.)