May Day events in New York, Chicago, Los Angeles, Boston, Milwaukee and Madison, Wisconsin were considerably smaller than in past years, as some of the hope for reform among Democrats that helped put Obama in the White House has faded.
In New York, demonstrators called for immigration reform as well as support for environmental causes, tax reform and changes in U.S. defense policy.
Demonstrators carried signs reading "Legalization now, stop the deportations" and chanted "We are people, we are not illegal" as they marched down Broadway.
Some carried signs pairing Obama with former President George Bush. "Obama you are not better than Bush," read one sign.
Immigration reform was front and center in Chicago, where about 300 demonstrators gathered on the city's west side before marching downtown.
Immigration reform is an issue of concern to Hispanic voters, whose support Obama needs for reelection in 2012.
Marchers in New York and Chicago were largely upbeat, blowing horns, drumming and chanting. They represented groups including domestic workers, environmentalists, gays and lesbians and even street vendors who they say are subject to unjust arrests.
One year ago Sunday, a vendor in Times Square helped avert disaster when he alerted police to a smoking vehicle that turned out to be a car bomb.
Police reported no incidents at either event.