Seoul planned to install a Terminal High-Altitude Area Defense system in its southern county of Seongju this year, which Beijing claims could hurt its security interests.

On the first day of the visit, the lawmakers including former Incheon Mayor Song Young-gil are set to meet with Chinese Foreign Minister Wang Yi and attend a dinner hosted by his ministry.

The next day, they plan to meet with experts and officials from China's Communist Party. On Friday, they plan to hold separate meetings with South Korean Ambassador to China Kim Jang-soo, members of the Foreign Affairs Committee under China's National People's Congress, and South Korean correspondents in Beijing.

Conservatives have called on them to cancel the visit, arguing Beijing could exploit the occasion for its aggressive campaign to derail the THAAD deployment. They said South Korea must not allow a foreign government to influence its sovereign decision on national defense.

On Tuesday, China's Foreign Ministry spokesman Geng Shuang welcomed their visit, while reiterating China's opposition to the deployment.

South Korea's ruling Saenuri Party and offshoot New Conservative Party for Reform condemned the visit, calling the move a form of "kowtowing."

"The reason China is meeting lawmakers from the Democratic Party is to have South Korea withdraw the plan to deploy the THAAD battery, especially as it counts presidential hopeful Moon Jae-in as a member," Rep. Jeong Yong-ki, Saenuri's spokesman said.

"(The Democratic Party) should not insult the South Korean people by engaging in a form of humiliating diplomacy," Jeong added.

Saenuri said Moon should come up with alternative options to THAAD, other than just criticize the deployment.