Landline and cellphone networks were also crippled in parts of the country, especially where fighting was fiercest, anti-regime activists said, warning the cutoff might mean authorities were planning to escalate their crackdown against the country's raging uprising.
Only residents with satellite connections to the Internet could access the Web, activists said.
The Internet has been a strategic weapon for both the rebels and the regime of President Bashar Assad, letting activists organize and communicate but also exposing them to surveillance, The New York Times reported.
U.S. network security company Renesys, which monitors and analyzes Internet disruptions, said 77 of Syria's 84 ISP address blocks, representing 92 percent of the country's routed networks, were unreachable.
It earlier said all 84 ISP address blocks were unreachable, "effectively removing the country from the Internet."
A nationwide shutdown of Web and voice communications would be unprecedented in the rebel uprising, although the regime has frequently shut down Internet and cellphone access in opposition-held areas -- sometimes for weeks at a time -- since the uprising began in March 2011, British newspaper The Guardian said.
Other Web-monitoring companies also reported the sudden disappearance of Internet traffic to and from Syria starting Thursday.
Online "hacktivist" group Anonymous said it determined after an "exhaustive analysis" the Assad regime had severed fiber-optic and coaxial cables going into Syria.
"Essentially, they have physically 'pulled the plug out of the wall,'" Anonymous said in a statement. "As we discovered in Egypt, where the dictator [President Hosni] Mubarak did something similar -- this is not damage that can be easily or quickly repaired."
The Assad regime's own websites are hosted in foreign countries, including the United States.
Anonymous threatened to remove all regime "Web assets" in an "Operation Syria" campaign that would first target Syrian embassies, beginning with the embassy in China, a Syrian ally.
The website was still up but appeared to be running slowly early Friday, a United Press International check indicated.
State-run Syrian TV said the Syrian Internet outage was not nationwide and was caused by a technical failure that technicians were fixing.
But Syria's information minister was quoted by the pro-regime Syrian al-Ikhbariya satellite TV channel as saying "terrorists ... targeted the Internet cable, which caused an interruption of the service in several Syrian cities."
The Assad regime has insisted it is fighting foreign terrorists, including some from al-Qaida, who want to replace his secular regime with an Islamic state that will widely persecute minority communities.
Syrian activists dismiss the charge as propaganda, insisting their campaign is nationalistic and aimed solely at removing a vengeful regime.
Fierce fighting near Damascus International Airport, southeast of the capital, coincided with officials shutting down the airport.
Activists said it was closed because of the fighting, which the Syrian Observatory for Human Rights opposition group, based in London, said included a fierce regime offensive.
State media said the airport was closed for maintenance.
The fighting Thursday had already prompted several airlines to cancel flights into Damascus.
The clashes increased after the Internet went down, the Syrian Observatory said, and included regime aerial-shell bombardments of at least 10 targets, the opposition Local Coordination Committees of Syria said.
The Syrian Network for Human Rights said it documented 127 deaths in Syria Friday. The victims included 5 children and nine women the report said.
The organization said it counted 64 deaths in and around Damascus, 22 in Aleppo, 17 in Daraa, 10 in Deir Al Zor, seven in Idlib, five in Homs and two in Hama.
Austria Presse Agentur said four Austrian soldiers assigned to a U.N. peacekeeping force in the Golan Heights were wounded Thursday afternoon when their 100-person convoy was attacked as it went to the airport.
Two of the four soldiers were sent to an Israeli hospital, while the other two, whose injuries were less serious, were flown to Austria, the news agency said, quoting a Defense Ministry spokeswoman.
Austria said it would protest the attack before the U.N. Security Council.
The Free Syrian Army umbrella group of militias said it launched a big push toward Damascus.
Regime forces were heavily deployed and appeared dug in for a fierce defense of the city, The Guardian said.
Syrian TV said regime forces battled "al-Qaida elements" in the Damascus suburbs.
Regime rocket fire reverberated from the city center overnight, The Guardian said.
Rebel and regime forces also engaged in deadly firefights in northwestern Syria near Aleppo, 190 miles from Damascus, with at least 10 rebel fighters killed Thursday night, the Syrian Observatory said.
Meanwhile, the U.N.-Arab League special envoy said in New York the Assad regime has to go.
"I think it's very, very, very clear that the people of Syria want change, and real change, not cosmetic changes," Lakhdar Brahimi told reporters after briefing the Security Council on the situation.
"The new Syria will not look like the Syria of today," he said.