The Denver Post newspaper reported that problems with computerized voting machines began at some polling stations when they opened at 7 am local time (1400 GMT).
Hundreds of voters lined up to vote, with CNN showing some lines wrapping around several city blocks.
Other jurisdictions also had problems operating the machines, and poll hours were also extended in some areas of Indiana, Pennsylvania and Illinois, CNN reported.
This year marks a federal deadline for change to modern technology, with New York and Connecticut the last in the country to use antiquated mechanical lever machines for a final time on Tuesday.
