In Ohio, more than 20 different vigils, rallies, marches and other events were planned.
At the American Petroleum Institute in downtown Washington, dozens of protesters held signs reading "Out of Iraq" and "No war, no warming," and chanted "No blood for oil!"
Craig Etchison, 62, a retired college professor and Vietnam veteran, said he has been protesting the war for years.
"I've watched with horror as Bush has lied about this war," he said in front of the building. "I'm appalled at the number of civilians we've killed, just as we did in Vietnam."
College students from the states New Jersey to North Dakota have planned walkouts, while students at the University of Minnesota vowed to shut down military recruiting offices on campus.
A protester wearing a mask marches in front of the White House to protest the war in Iraq in Washington, DC. US President George W. Bush on Wednesday defended the launch of the war against Iraq five years ago, vowing no retreat as he promised US forces would triumph over Iraqi insurgents. [Agencies]