Kim Jong Un's "traitor" uncle is believed to have been executed by machine gun fire, after being accused of plotting against the regime, as well as partaking in gambling and womanising.
North Korea said in a statement via the state news agency on Thursday night that a military tribunal had ordered the execution of Jang Song Thaek, the first public killing of a senior official in many years.
In a message displayed on billboards and played in metro stations, Pyongyang declared the bespectacled man once considered the second most powerful official in North Korea "a traitor to the nation for all ages who perpetrated anti-party, counter-revolutionary factional acts in a bid to overthrow the leadership of our party and state and the socialist system.
Television moitors displayed at the Yongsan electronic market shows the news of Jang Song-thaek's excution in Seoul
A specially-convened military tribunal ruled Jang "human scum" and a "wicked political careerist, trickster and traitor for all ages in the name of the revolution and the people and ruled that he would be sentenced to death according to it.
"The decision was immediately executed."
Yonhap News Agency reported that South Korea's government has called an urgent security meeting, led by defence chief Kim Jang-soo, over the potential security ramifications.
South Korean President Park Geun-Hye said the South feared a wave of purges would follow, as 30-year-old Kim seeks to further cement his leadership.
"North Korea is now engaged in a reign of terror while carrying out a massive purge to consolidate the power of Kim Jong-Un," she told a cabinet meeting.
North Korea Strategic Information Service Centre, translated by NKNews, quoted an unnamed source as saying the execution was not Kim Jong Un's decision alone - but encouraged by Jang's wife Kim Kyong Hui, and arranged by Kim Jong Un's elder brother Kim Jong Chul, who headed up the corruption investigation.