"If this was happening in every city, there would either be martial law or an end to war," said one Berkeley student who chained himself to 16 others on a major San Francisco street.
Protests took place in other cities across the United States as well as in European capitals. During morning rush hour in the Washington D.C., more than 100 demonstrators temporarily shut down the Key Bridge, a major route from Virginia into Washington's Georgetown neighborhood.
Three people were arrested there, but the rest dispersed peacefully after police asked them to leave, officers said.
More than 100 protesters later gathered in pouring rain on the streets by Lafayette Park near the White House.
"We're the youth of America and we're saying that we don't want this war in our name," said Peter Matthews, 17.
In New York, which took the brunt of the September 11, 2001 attack that President Bush has repeatedly cited as an example of the threat to America, "September 11th Families for Peaceful Tomorrows" condemned what they called an illegal and immoral U.S. war.
"We do not want other innocent families to suffer the trauma and grief that we have endured," the group said.