Internet traffic into and out of the country was blocked.
Syrian residents also reported that mobile and telephone lines were disrupted, according to Reuters.
Google also reported that Syrians were unable to access any of its services Thursday.
The Syrian government is blaming "terrorists" for the communication blackout, according to the BBC.
"The terrorists targeted the Internet lines, resulting in some regions being cut off," Syria's minister of information said on a pro-government TV station, according to the report.
But forces trying to overthrow President Bashar Assad have used Internet sites like YouTube, Facebook and Twitter to post information about their fight.
While this isn't Syria's first such shut down, it is the worst blackout since the Syrian uprisings began over a year ago, Reuters said.