Seoul, South Korea (CNN) Parts of a US-built anti-missile system designed to mitigate the threat of North Korean missiles have been moved to the planned deployment site in South Korea as tensions with the nuclear-armed country escalate.

Trucks hauling components of the Terminal High Altitude Area Defense (THAAD) system rolled into the site in North Gyeongsang province, according to a statement from the South Korean Defense Ministry on Wednesday.

"Both South Korea and the United States have been working to secure the operational capacity of the THAAD system in preparation for North Korea's advanced nuclear-missile threat," the statement said.

"Therefore, this measure was to secure operational capacity by placing some parts of the available THAAD system at the deployment site."

The missile system has angered North Korea and also drawn sharp opposition from China, which sees it as a threat to its own security.

"We have expressed serious concern to the US and South Korean sides," Chinese Foreign Ministry Spokesman Geng Shuang said Wednesday.

"The US-South Korean deployment of THAAD in South Korea will harm strategic balance in the region and further stimulate tensions on the Korean Peninsula.

"China strongly urges the US and South Korea to stop actions that would raise regional tensions as well as harm China's strategic and security interests by canceling the THAAD deployment and withdrawing relevant equipment."

He added that China will "firmly take necessary measures to safeguard its own interests."

In Seongju county, at the location of the THAAD site, around 4,000 police were present to ensure the equipment's delivery.

Around 400 protesters were present at a demonstration near the site, and police in riot gear held back protesters as the equipment rolled past on military trucks.

Hwang Soo-young, an activist with the government watchdog group, the People's Solidarity for Participatory Democracy (PSPD), was at the site of the protest Wednesday morning. She claimed that the protests turned violent as "police were pushing residents away."

She claimed six people were injured during the encounter, although CNN has not been able to independently verify the claim.

She said that vehicles with equipment "including radar, launchers and generators" started passing the village of Soseongri at around 4.45 a.m. (3.45 p.m. Tuesday ET).

Local complaints center around the lack of consultation over the decision to deploy the missile system near their homes. The voices of local people were "never heard, they never asked these people," she said.

The goal is to have the complete system fully operational by the end of this year but the US and South Korea have publicly stressed the need to speed up the deployment of the technology as tensions have mounted with Pyongyang.

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And later on Wednesday, the White House will hold an unusual briefing on North Korea, with Secretary of State Rex Tillerson, Defense Secretary James Mattis and other officials outlining the threat for the entire Senate.

Kim Jong Un guides live fire drills in Wonsan, North Korea, on April 25, to mark the 85th anniversary of the the founding of the Korean People's Army.

Anti-missile system

THAAD is designed to shoot down incoming short, medium and intermediate ballistic missiles that threaten civilian populations, just the type of weapons North Korea claims it has.

Each THAAD system is composed of five major components: interceptors, launchers, a radar, a fire control unit and support equipment, according to Lockheed Martin, the security and aerospace company that serves as the prime contractor for the equipment.

"Deploying THAAD is a critical measure to defend the ROK (Republic of Korea) people and Alliance forces against North Korean missile threats, as highlighted by the recent ballistic missile launches by North Korea," a statement from the office of the US Secretary of Defense said Tuesday.

"North Korea's unlawful weapons programs represent a clear, grave threat to US national security in the United States, the ROK and Japan."

Opposition at home

The announcement to deploy THAAD has also faced opposition from many residents of Seongju county, near the deployment site, and criticism of the decision to deploy it -- against the backdrop of the increased militarization of the Korean Peninsula -- was a key part of protests that helped bring down former President Park Geun-hye.

"The decision made by the government to deploy THAAD was not democratic at all," said Baek Ga-yoon, coordinator for the Center for Peace and Disarmament, which advocates for a nuclear-free Korean Peninsula.

She accused acting-President Hwang Kyo-ahn of taking advantage of the political instability around Park's impeachment to press ahead with THAAD's deployment "without any agreement from the National Assembly and the villagers of Seongju."

The upcoming election is expected to result in a swing to the left, likely in favor of the Democratic United Party's Moon Jae-in, who narrowly lost to Park in 2012 and has led opinion polls since her ouster.

Moon's party has been critical of the THAAD agreement and suggested it should be renegotiated, saying Park should have sought the approval of the National Assembly before deployment began.

"Presidential candidate Moon Jae-in has consistently stated that the deployment of THAAD should be decided by the next government through taking into account sufficient public consultation, consensus and consideration of our national interests and the ROK-US alliance," a statement released by Moon's spokesman, Park Kwang-on, read.

"It is better to discontinue the deployment of the equipments and pass the final decision to the next government after going through public and national consensus and consultation between ROK-US."