Thae Yong Ho defected to South Korea in August last year and since December 2016 has been speaking to media and appearing on variety television shows to discuss his defection to Seoul and his life as a North Korean envoy.
Mr Yong Ho said during his first news conference with foreign media: "When Kim Jong Un first came to power, I was hopeful that he would make reasonable and rational decisions to save North Korea from poverty, but I soon fell into despair watching him purging officials for no proper reasons.
"Low-level dissent or criticism of the regime, until recently unthinkable, is becoming more frequent.
The North Korean elite are outwardly expressing their discontent towards young leader Kim Jong Un
"We have to spray gasoline on North Korea, and let the North Korean people set fire to it."
Low-level dissent or criticism of the regime, until recently unthinkable, is becoming more frequent
Thae Yong Ho
Mr Yong Ho, 54, has said publicly that dissatisfaction with Kim Jong Un prompted him to flee his post.
Two university-age sons living with him and his wife in London also defected with him.
North and South Korea are technically still at war because their 1950-53 conflict ended in a truce, not a peace treaty.
Thae Yong Ho defected to South Korea in August last year
Inside North Korea: The pictures Kim Jong-un doesn't want you to see
Thu, December 15, 2016
Since 2008, photographer Eric Lafforgue ventured to North Korea six times. Thanks to digital memory cards, he was able to save photos that was forbidden to take inside the segregated state
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Taking pictures in the DMZ is easy, but if you come too close to the soldiers, they stop you
The North, which is subject to UN sanctions over its nuclear and missile programmes, regularly threatens to destroy the South and its main ally, the United States.
Mr Yong Ho is the most senior official to have fled North Korea and entered public life in the South since the 1997 defection of Hwang Jang Yop, the brains behind the North's governing ideology, "Juche", which combines Marxism and extreme nationalism.
Today's North Korean system had "nothing to do with true communism", he said, adding that the elite, like himself, had watched with unease as countries like Cambodia, Vietnam and the former Soviet Union embraced economic and social reforms.
North and South Korea are technically still at war because their 1950-53 conflict ended in a truce
Mr Yong Ho has said that more North Korean diplomats are waiting in Europe to defect to South Korea.
North Korea still outwardly professes to maintain a Soviet-style command economy, but for years a thriving network of informal markets and person-to-person trading has become the main source of food and money for ordinary people.
Fully embracing these reforms would end Kim Jong Un's rule, Mr Yong Ho said.
Mr Yong Ho has said that more North Korean diplomats are waiting in Europe to defect to South Korea
Kim Jong-Un in pictures
Mon, January 23, 2017
Kim Jong-Un, The Supreme Leader of North Korea takes himself and his leadership very seriously.
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North Korean leader Kim Jong-Un paying his final respects at the bier of Kang Ki Sop, alternate member of the Central Committee of the Workers' Party of Korea, deputy to the Supreme People's Assembly of the DPRK and general director of the General Administration of Civil Aviation, following Kang's death at an undisclosed location in North Korea.
Asked if Kim Jong Un's brother, Kim Jong Chol, could run the country instead, he remained sceptical.
Mr Yong Ho said: "Kim Jong Chol has no interest in politics. He is only interested in music.
"He's only interested in Eric Clapton. If he was a normal man, I'm sure he'd be a very good professional guitarist".