Beginning in 2002, and continuing after the 2003 invasion of Iraq, large-scale protests against the Iraq War were held in many cities worldwide, often coordinated to occur simultaneously around the world. After the biggest series of demonstrations, on February 15, 2003, New York Times writer Patrick Tyler claimed that they showed that there were two superpowers on the planet, the United States and worldwide public opinion.

These demonstrations against the war were mainly organized by anti-war organizations, many of whom had been formed in opposition to the invasion of Afghanistan. In some Arab countries demonstrations were organized by the state. Europe saw the biggest mobilization of protesters, including a rally of three million people in Rome, which is listed in the Guinness Book of Records as the largest ever anti-war rally.

According to the French academic Dominique Reynié, between January 3 and April 12, 2003, 36 million people across the globe took part in almost 3,000 protests against the Iraq war.

In the United States, even though pro-war demonstrators have been quoted as referring to anti-war protests as a "vocal minority", Gallup Polls updated September 14, 2007 state, "Since the summer of 2005, opponents of the war have tended to outnumber supporters. A majority of Americans believe the war was a mistake."