The march was one of one of several held today around the United States and the world in the run-up to March 20, the fourth anniversary of the U.S.-led invasion of Iraq.
The march began near the Vietnam War Memorial, blocks from the White House, and proceeded across the Potomac River toward the Pentagon.
A smaller prowar group followed the antiwar protesters. Police kept the two groups apart.
Police in Los Angeles said they were expecting between 5,000 and 10,000 protesters at an antiwar rally.
Demonstrations were also held today in Turkey, Greece, and Spain.