Ms Park took office in Seoul earlier this year as North Korea conducted its third nuclear test, enraging world public opinion, and threatening to engulf its southern neighbour and its ally, the US, in a war.
She told a cabinet meeting yesterday: "North Korea is carrying out a reign of terror, undertaking a large-scale purge in order to strengthen Kim Jong Un's power.
"From now on, South-North Korea relations may become more unstable."
In her usual carefully scripted manner, the president called for vigilance to safeguard the wealthy South's achievements.
Jang Song Thaek, the uncle of North Korean ruler Kim Jong Un, was dismissed from his posts for "criminal acts" ranging from mismangement, corruption and leading a "dissolute and depraved life", state media said on Monday.
Television in the tightly controlled and impoverished Communist state showed him being frogmarched by uniformed personnel out of a meeting of the ruling Workers' Party.
Associates of Mr Jang are believed to have been executed in the purge of a man once viewed as a regent for Kim Jong Un, who is aged about 30 and the third of his family to run the country.