Demonstrations have been held on the South Korean island of Jeju to protest at construction of a new naval base.
Tensions have been rising over the past week as police moved in to the site to remove protesters who have been blocking access since June.
The government says the $970m (£600m) development is vital for national security.
But residents and activists argue that the base could spark regional tensions and is damaging to the environment.
More than 1,000 riot police dispersed protesters on Friday, allowing naval engineers to resume their work.
"Heavy equipment moved into the site after the navy erected fences to block off the protesters," a police spokesman in Seoul told AFP.
Earlier this week, the government warned that work could not be delayed any longer.
The BBC's Lucy Williamson in Gangjeong, where the site is, says the road there is lined with bright yellow flags with slogans voicing opposition to the project.
Rising above the small neat houses, the grey walls surrounding the military construction site are lined with riot police - 20 buses parked up in preparation for the event, our correspondent says.
The rally is being organised by a disparate crowd of religious leaders, political activists, and local groups, who want to see the project aborted.
They say the naval base - which will be open to US warships - risks creating tensions with China and causing environmental damage.
But the plan has supporters as well as detractors, with some local people saying the new project will bring more money into the area.
The base, which is due to be completed in 2014, is to host some 20 warships to protect shipping lanes through which most of South Korea's sea trade passes.