The US military on Wednesday started moving parts of its THAAD anti-missile defence system to a deployment site in South Korea.
The move came as tensions are running high in the region over North Korea's missile and nuclear programmes.
The earlier-than-expected move prompted protests by hundreds of local residents and was denounced by the frontrunner in South Korea's presidential election on May 9.
A spokesman for Moon Jae-in said the decision "ignored public opinion and due process" and demanded the deployment be suspended until the next administration was in place and had made its policy decision.
The United States and South Korea last year agreed to deploy the Terminal High Altitude Area Defense (THAAD) system to counter the threat of missile launches by North Korea.
However the move has angered China, which says the advanced system will do little to deter the North while destabilising the regional security balance.
South Korea's defence ministry said some elements of THAAD were moved to the site on what had been a golf course in the south of the country.
"South Korea and the United States have been working to secure an early operational capability of the THAAD system in response to North Korea's advancing nuclear and missile threat," the ministry said in a statement.
The battery is expected to be operational by the end of the year, it added.
Television footage showed military trailers carrying large units including what appeared to be launch canisters being driven into the planned THAAD battery site, about 250 kilometers (155 miles) south of Seoul.
Protestors block vehicles
Images showed local protesters hurling water bottles at the vehicles and police trying to block them.
The Pentagon said the deployment was a critical measure to defend South Korea and its allies against North Korean missile threats and it would complete it "as soon as feasible".
The US and South Korean militaries have been reluctant to publicly discuss the progress of the deployment ahead of the South Korean presidential election.
Moon, the favourite to win the race, has said the new South Korean administration should decide on whether to deploy the THAAD after gathering public opinions and having further discussions with Washington.
‘Largest-ever’ military drill
North Korea's KCNA news agency said on Wednesday leader Kim Jong Un had supervised the country's "largest-ever" live-fire drill to mark the 85th founding anniversary of its military.
The drill came instead of a nuclear test or the launch of a long-range missile as feared amid pressure from the United States and China, its sole major ally which has been irritated by Pyongyang's weapons development.
"The brave artillerymen mercilessly and satisfactorily hit the targets and the gunshots were very correct, he said, adding that they showed well the volley of gunfire of our a-match-for-a-hundred artillery force giving merciless punishment to the hostile forces," KCNA said.
A US submarine designed to carry 150 Tomahawk cruise missiles entered a South Korean port on Tuesday as the USS Carl Vinson carrier group steamed towards the Korean waters in an effort to deter the North from a sixth nuclear test and more missile launches.
South Korea's navy has said it plans to hold a joint drill with the US strike group late this month.