South Korean police are investigating a hacking attack on an internet provider that brought down the servers of three broadcasters and two major banks.
The army also raised its alert level due to concerns of North Korean involvement.
The network provided by LG Uplus Corp showed a page that said it had been hacked by a group calling itself the "Whois Team", an unknown group.
It featured three skulls and a warning that this was the beginning of "Our Movement".
Servers at television networks YTN, MBC and KBS were affected, as well as Shinhan Bank and NongHyup Bank.
Police and government officials declined to speculate on whether North Korea was behind the hack.
North Korea has in the past staged cyber-attacks on the world's most wired country, targeting conservative newspapers, banks and government institutions.
South Korea's military said it was not affected but raised its state of readiness in response.
None of South Korea's oil refineries, power stations, ports or airports were affected.
The biggest attack by Pyongyang was a ten-day denial of service attack in 2011 that antivirus firm McAfee, part of Intel Corp, dubbed "Ten Days of Rain" and which it said was a bid to probe the South's computer defences in the event of a real conflict.
Shinhan Bank, one of the financial institutions affected in the latest attack, said its servers were back up by 7am Irish time.
Local shares extended losses following the initial reports of outages to end down 1%.