Seoul (CNN) -- Tensions soared on the South Korean island of Jeju on Thursday as hundreds of residents, activists and priests protest against the building of a naval base.
About 500 supporters of the project also arrived Thursday on the second day of key construction work.
Crews have blown up rocky areas with dynamite to prepare for a caisson and other structures that will help with the construction of the docks.
Protests against the building of the naval base started seven years ago over fears of damage to the environment and nature on the island. Protesters say it would also threaten the peace on the island, parts of which are UNESCO world heritage sites, and affect tourism.
"Activists are trying to approach the construction area on kayaks and the marine police is blocking them so the construction company can blast the rocky area," Jeju resident Hong Ki-ryong said. "The naval base on this Island can create higher tension in North East Asian region."
Jeju Island Gov. Woo Keun-min urged the central government to suspend the contentious naval project, calling for a public hearing on the case. But the South Korean navy vows to continue the project.
It claims the harbor, big enough to accommodate 20 battleships and two cruise ships, is necessary to strengthen its naval defenses in the South Sea.
In a statement, it said it has reviewed "environmental issues ... the procedural validity of the project. We have confirmed that there are no significant problems arising from these issues." The government says former land owners have been compensated.
Opponents fear the South Korean naval base will also be used by the United States, which has 28,500 military personnel based on the mainland.
The project has also attracted celebrity criticism. Actor and environmental activist Robert Redford claims the project is due to an arms race, saying in an article "It seems motivated by the United States' urge to encircle China with its Aegis anti-ballistic system... and by the South Korean navy's construction of a massive naval base for aircraft carriers, submarines and destroyers to carry Aegis."
Journalist Yoonjung Seo contributed to this report.