The protests came as the news media reported that April was the deadliest month for US soldiers in Iraq this year, with 69 killed as of Friday.
The marchers included Oscar-winning actress Susan Sarandon, former Democratic presidential candidate Jesse Jackson and anti-war campaigner Cindy Sheehan, whose son died in Iraq.
“We have lost money and lives and honour,” Mr Jackson said, urging Americans to join the marches around the country. “We need to march to end a war based upon lies and spies.”
Protest organisers said another aim was to warn against any US military intervention in Iran over the nuclear standoff with Tehran.