The summit was intended to cement improved relations after a frosty period of tension between China and South Korea.
But just hours before the Chinese leader treated his South Korean counterpart to a red carpet welcome in Beijing, a journalist from Korean president Moon Jae-in's press pool was beaten bloody at a bilateral trade event.
The Chinese, who rarely acknowledge mistreatment of journalists, expressed concern over the incident
South Korea's foreign ministry called for an investigation into the incident. Ministry spokesperson Roh Kyu-deok said "our government immediately expressed concern to the Chinese government."
A spokesperson for Korea's largest opposition party went several steps further, demanding Moon immediately cancel his visit to China.
"Assaulting the members of the press pool who accompanied the head of state in the middle of a state visit is an act of terror against South Korea," said Jang Je-won of the Liberty Party.
The beating took place at a trade event attended by Moon on Thursday called the Korea-China Economy and Trade Partnership. It wasn't immediately clear what led to the incident or who was responsible.
Video broadcast from multiple camera angles show a crowd of men in suits surrounding and man-handling Lee Chung-woo, a reporter from the Seoul-based Maeil Business Newspaper.
One of the men in suits can be seen kicking Lee after he falls to the ground. Off camera, Korean journalists can be heard yelling in English and Korean: "Don't touch camera" and "Who's in charge of the Chinese security?"
Later, Lee can be seen, his face bloody and bruised, standing not far from an exhibit with the sign K-Pop, which ran flashy video of a Korean boy-band performing on stage.
An official from the Korean president's office confirmed to CNN that Lee was later taken to a hospital for treatment.
Lu Kang, a spokesperson for China's foreign ministry, later told journalists "If anyone was indeed hurt, then of course we express concern."
Lu said South Korea and China had both put great effort into ensuring that the summit would be a success.
The Chinese rarely acknowledge mistreatment of journalists working in China.
China is South Korea's largest trading partner. The two countries established formal diplomatic relations in 1992.
But ties dramatically deteriorated after Moon's predecessor agreed to accept the installation of a US missile defense system.
Beijing argued that the system, known by the acronym THAAD, poses a threat to China's national security.
China appeared to show its displeasure, with a 50% drop in Chinese tourists traveling to South Korea. A South Korean car-maker and a supermarket chain also experienced a drop in their sales in China in the months after the THAAD announcement.
In an interview with Chinese state television ahead of his visit, president Moon insisted that the THAAD system poses no threat to China. He argued the missile defense system was necessary to protect his country from North Korea and its nuclear weapons program.
Pyongyang has fired at least 23 missiles during a ten month period this year. All of these missile launches are banned under multiples United Nations Security Council resolutions.
Moon appeared to be trying to exercise Korean soft power during his visit to China. He was to be accompanied to a state dinner in Beijing Thursday night by Song Hye-kyo, one of Korea's most popular actresses.
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