Tension has run high since North Korea conducted its fourth nuclear test in January and followed up with a satellite launch and a string of test launches of missiles.
South Korea announced in July that a US THAAD anti-missile defense unit would be deployed in Seonjgu, but residents have protested, citing safety fears over the system's sophisticated radar and its potential to be a wartime target, Reuters reported.
The plan has also angered China and prompted a North Korean warning of retaliation.
Seongju residents, many of them farmers cultivating a melon variety that has brought the county domestic fame, sat in somber silence as they had their heads shaved while a protest leader led a crowd in chants of "No THAAD!"
"THAAD should not be deployed at all, not just in Seongju, but anywhere in South Korea," said Yoo Ji-won, a 63-year-old melon farmer. "We residents gathered here and shaved heads to demonstrate against its deployment."
As many as 908 people joined in the head-shaving part of the protest, organizers said.
"This is the most powerful way of displaying protest," protest leader Kim An-soo said. "We cannot protest any bigger".
The South Korean Defense Ministry has vowed to minimize any impact from THAAD on residents and the environment.
"THAAD is a self-defensive measure we've decided to deploy to protect the lives of our people," South Korean president Park Geun-hye said in a speech on Monday.