Voting was extended in at least two states following computer breakdowns that prevented some voters from casting their ballot.
In the eastern state of Pennsylvania, where balloting was extended in two counties for an extra hour officials said a programming error in the voting machines had caused delays and forced some voters to use paper ballots.
"At least one machine at each polling station was mis-programmed," Elaine Ludwig, the chief clerk of elections in Lebanon County, said.
"I am so up to my ears trying to get this thing rectified," she added.
Voting hours were also extended in Delaware County, in the Midwestern state of Indiana, because of glitches with the electronic voting machines at 75 precincts, an official told news agency AFP.
Sporadic problems were also reported in Colorado, Illinois, Maryland, New Jersey, Tennessee and Utah.
In Colorado's Denver County, a power outage led to a computer breakdown that prompted the mountain state's Democratic Party to seek a court order to extend voting. A judge, however, denied their request.
Doug Chapin, director of Electionline.org, a non-partisan group that tracks election changes, said though he expected glitches he was surprised there were so many.
"We were expecting these kinds of problems but I am a little bit surprised that ... they are in so many different places," he said.
Mr Chapin warned that some of the computer problems could be used to challenge the result in tightly contested races.
"Anytime you have these kinds of problems, at least theoretically, they can be used as a basis for a challenge," he said.
In Florida's Broward County, notorious for the "hanging chad" debacle in the disputed race that saw President George W Bush triumph over Al Gore in 2000, new problems were reported.
In one precinct, 14 voting machines stopped working for 90 minutes, prompting some voters to walk away. Service was later restored.
In one elementary school in the predominantly African American district of East Cleveland, Ohio, all 12 machines went down for two hours when voting opened.
Polling problems
A voter watchdog group said it had received thousands of calls from people complaining about problems at the polls, mostly with electronic machines.
"Complaints are all over the map," Mary Boyle, spokeswoman for the Washington-based Common Cause.
In one polling station in Friendship Heights, Maryland, just outside Washington, one of the 16 voting machines failed as soon as the polls opened.
Others worked well, but did not assuage voter concerns about whether their vote cast by electronic machines was safe.
"They are very user-friendly and there is good support, but I still wish they had a printout," voter Chris Strom told news agency AFP after casting her ballot.
But voter Donald Rollert said he had no problem with the computers. "It was a piece of cake" he said after voting.
At a polling place nearby things were also going well, after fears that problems with voter cards experienced during primary elections in September could resurface.
"It went very smoothly. Much better than last time ... where I was disenfranchised for the first time in 40 years", said Jeff Ross, 56.
"The only question is, 'Does your vote exist?'"
At another polling station, even voters well into their 80s had no problems with computerized systems -- though several used large magnifying glasses.
Both Republicans and Democrats deployed battalions of lawyers and poll watchers to observe the vote, especially in areas where problems were expected.
The Department of Justice also dispatched 800 lawyers to some jurisdictions "to ensure that everything with regard to federal laws is being complied with".