The Lincoln Memorial in Washington was vandalized with red spray paint.
The graffiti found on a pillar at the monument appeared to say "f--- law," the National Park Service told local station NBC 4 News.
A preservation crew is already working to remove the graffiti, which was discovered early Tuesday morning, the NPS said.
The crew is applying a "mild, gel-type architectural paint stripper" to remove the red paint without damaging the underlying stone, according to the report.
The park service is still investigating the matter.
The vandalism comes amid a public debate over Civil War statues and memorials honoring the Confederacy.
White supremacist groups converged on Charlottesville, Va., to protest the removal of a state of Confederate Gen. Robert E. Lee on Saturday, which led to violence.
One person was killed and at least 19 were wounded after a man who took part in the white supremacist rallies drove his car into a crowd of counterprotesters. The alleged driver, James Alex Fields Jr., was arrested and charged with second-degree murder.
Protestors toppled a Confederate statue in Durham, N.C., on Monday night. And in Boston, police arrested a suspect accused of vandalizing the city’s Holocaust memorial.