Rep. Park Sun-young of the conservative minor Liberty Forward Party was sitting in a chair in front of a church and listening to a North Korean defector's speech when she suddenly fainted, an Yonhap News Agency photographer said.
Police confirmed Park was rushed to Seoul National University Hospital by ambulance.
Later in the day, Park said in a press release that another four North Koreans were seized by the Chinese authorities near their border, including two parents with a 20-day-old infant and a young man in his 20s.
Park added that all of them have families in South Korea, saying "it is unthinkable to have a baby that young and the mother sent back arrested."
Park had been staging a hunger strike in front of the Chinese mission for 11 days as of Friday to urge Beijing not to return dozens of defectors detained in China to their communist homeland, where they could face harsh punishments and even death.
As a key ally of North Korea, China usually repatriates defectors from North Korea, seeing them as economic migrants, not refugees.
A spokesman from the Seoul National University Hospital said in regards to Park's condition that "she is stable and only dehydrated," adding she will be treated according to the progress of her health.
The hospitalization occurred the same day Chinese Foreign Minister Yang Jiechi held talks with his South Korean counterpart in Seoul on the issue of North Korean defectors arrested in China, as well as on other bilateral issues.
Yang, who came to Seoul on Thursday, paid a courtesy call on South Korean President Lee Myung-bak. Presidential spokesman Park Jeong-ha said Lee asked for active cooperation from China to "smoothly" resolve the defector issue.
Yang responded that China will take South Korea's request seriously and relay what was discussed in the meeting to Chinese President Hu Jintao, according to the spokesman.
Seoul previously sought what is dubbed "quiet diplomacy" with Beijing to settle the issue of North Korean defectors, but has taken an unusually tough stance on the latest cases amid mounting domestic pressure to save the North Koreans.
A stream of North Koreans continue to flee their homeland to avoid chronic food shortages and harsh political oppression. Most cross the border into China for eventual resettlement in South Korea, home to more than 23,000 North Korean defectors.