Nearly 50,000 telephones at Olympic sites and parts of Athens went dead for more than 10 hours Thursday, officials said.

The outage follows a July 12 electricity blackout across southern Greece that raised concern over the stability of public utility networks ahead of the Aug. 13-29 Olympic Games (search).

The phones went down during attempts to upgrade the system, officials said.

"Some 48,000 fixed-line phones stopped working during work to improve a telephone switching center," Christos Malapanis, an official at the Greek Telecommunications Organization (search), or OTE, told state-run NET television.

Company officials told The Associated Press that sites affected included the Olympic Village, which opened last week, a media village, and four other games facilities. A back-up network kept about half the telephones working at the affected sites, the officials said.

OTE and Greece's Public Power Corporation are both Olympic sponsors.