SEOUL (Reuters) - South Koreans protesting against a plan to deploy a U.S. missile defense system in their district blocked a minibus carrying the prime minister for several hours on Friday, preventing him from leaving an office.

The government announced on Wednesday that the southeastern county of Seongju had been chosen as the site for a Terminal High Altitude Area Defence (THAAD) battery aimed at countering what it sees as the threat from North Korea's missile and nuclear program.

But residents of the melon-farming area said they were not consulted and they do not want the missile defense unit, due to be deployed by late next year.

Protesters threw eggs and plastic water bottles at Prime Minister Hwang Kyo-ahn as he spoke on the steps of the county office to apologize for not briefing residents earlier, TV footage showed.

Security guards used boards and umbrellas to shield Hwang and he was quickly taken inside the building.

When he tried to leave the compound, a crowd of several hundred with a tractor blocked his vehicle.

Hwang later got out of the bus and into a car but it too was trapped in the crowd, Yonhap news agency reported. He then moved to another car and left, Yonhap said.

Seongju residents refused to accept the government's explanation that the site was chosen because it would have no impact on the safety of residents and the environment.

China has also protested against the planned deployment of the THAAD system, which has a powerful radar it worries can see into its territory.

North Korea's military on Monday threatened to retaliate against the deployment of the system with a "physical response" once its location and the timing of its installation were decided.

Tension on the Korean peninsula has been high since North Korea conducted its fourth nuclear test in January and followed that with a satellite launch and a string of test launches of various missiles.

In South Korea, the prime minister holds a largely ceremonial role as the head of the cabinet in a powerful presidential system. He and the cabinet are appointed by the president who serves a single five-year term.

(Reporting by Jack Kim; Editing by Tony Munroe, Robert Birsel)