Voters calling in to an election-day hotline reported more than 1,100 problems with the ATM-like machines, from improperly tallied choices to frozen screens that left their votes in limbo.
Voters in Maryland said congressional candidates were left off ballots, while some in Florida told hotline volunteers that their ballots had already been filled out when they stepped up to vote, watchdogs said.
Machines in New Orleans, Miami and suburban Philadelphia failed to start punctually in the morning, leading to long lines at polling places and prompting some to turn away from the polls, according to activists with the Election Protection Coalition.
The nonpartisan group said it had received 1,166 complaints as of late evening involving a wide array of machines.
"It gives us the uneasy feeling that we're only seeing the tip of the iceberg," said Cindy Cohn, legal director of the Electronic Frontier Foundation, a technology-policy group involved in the coalition.
Officials with voting-systems companies said most problems could be traced to human error, rather than the equipment.
"Everything we see and hear and talking to our members who are in turn in touch with election officials seems to be very positive," said Bob Cohen, a spokesman with the Information Technology Association of America, which counts voting-system vendors such as Diebold Inc. among its members.
About 45 million registered voters are expected to cast a ballot on touch-screen systems, which have been touted by election officials as a way to avoid a repeat of the messy recount battle touched off by antiquated punch-card systems in Florida four years ago.
Computer scientists say the machines are prone to the glitches and security holes all too familiar to home-computer users.
The controversy has prompted some states to postpone upgrades until after the election, even though the federal government has earmarked $3.9 billion for that purpose.