The U.S. troops in South Korea began installing the equipment for the Terminal High Altitude Area Defense (THAAD), an advanced missile defense system.
Six trailers reportedly carrying the high-profile X-band radar of the THAAD system, mobile launchers and other elements were seen entering the civilian-restricted zone starting around midnight, reports the Yonhap news agency.
The unannounced operation came just six days after the U.S. Forces Korea ( USFK) secured the land in Seongju, North Gyeongsang Province, from the South Korean government.
The allies signed a deal last year to deploy the THAAD system on the peninsula to help intercept North Korea's ballistic missiles.
South Korean Ministry of National Defense also confirmed the start of full-scale deployment.
"The measure this time is meant to secure early operational capability by positioning some available parts first at the site," said the ministry adding that the two sides will proceed with the environmental assessment and construction of relevant facilities as scheduled.
The allies plan to put the THAAD unit into full operation by the end of this year.
Deploying THAAD is a critical measure to defend South Korean people and alliance forces against North Korean missile threats, as highlighted by the recent ballistic missile launches by North Korea, the Pentagon said in a statement.
Meanwhile, hundreds of protestors clashed with the police against the transportation of the THAAD equipment.
They waved pickets reading "No THAAD, No War" and "Hey, U.S.! Are you friends or occupying troops?"
China has also opposed the deployment of the THAAD system in South Korea.
Designed to shoot down missiles at a high altitud, a THAAD battery is made up of six truck-mounted launchers carrying some 50 interceptor missiles, and a fire control and communications unit, which are linked to the very long-range X-band radar system.