A problem caused by a software upgrade has caused a widespread outage on AT&T's U-verse broadband service this week, leading to a torrent of attacks against the carrier on social media.

AT&T said on Wednesday that the outage had affected less than 1% of its subscribers, but customers in areas spread across the Southeast and Southwest had lost service.

U-verse is AT&T's set of data and TV services for homes and small businesses, which runs over fiber to neighborhood nodes. At the end of last year's third quarter, the carrier reported it had 7.4 million U-verse subscribers.

The U-verse Facebook page and Twitter stream were filled with criticism of AT&T, presumably from subscribers who found a way to get online without home broadband. Late Wednesday, many commenters said they had been without service for three days.

A status update on U-verse's Facebook page on Monday in honor of the Martin Luther King holiday became a sounding board for complaints about the failure, drawing more than 100 comments about loss of service.

"Yeah MLK day is great and everything but how about fixing my cable and Internet that has been out for the past two days? Maybe with a little sense of urgency since I'm paying you every month without ever being late," one commenter wrote on Tuesday. Others on the Facebook page and on Twitter complained about poor customer service and long waits on hold on the phone. Many demanded an estimate as to when service would be restored, which AT&T had not given by late Wednesday.

"We know we have a U-verse issue affecting less than 1% of our U-verse subscribers, but that is too many," AT&T said in a statement on Wednesday. "We are working hard to fix this and making progress -- the issue is related to servers. We still are working to determine when service will be completely restored. We sincerely apologize for this."

A later statement shed more light on the problem but didn't say when service would be restored.

"The issue impacting some U-verse subscribers has been tracked back to a software upgrade," AT&T said. "We continue to work to determine when service will be completely restored. Our continued apologies for the inconvenience."

Meanwhile, the service's @Uverse Twitter account was keeping up a steady stream of replies to customers' complaints, issuing brief but profuse apologies. "Hi Chris, we are so, so sorry for the inconvenience this service issue has caused, and we'll be in touch with updates," one of @Uverse's tweets read.

Stephen Lawson covers mobile, storage and networking technologies for The IDG News Service. Follow Stephen on Twitter at @sdlawsonmedia. Stephen's e-mail address is stephen_lawson@idg.com