Organizers said their "national day of action for immigration justice" included events in more than 140 cities in at least 39 states, with drum-banging and flag-waving masses chanting "Si se puede" -- "Yes we can" -- in rallies from coast to coast.

The events served as a visible demonstration of political clout for supporters of the nation's estimated 11-12 million undocumented immigrants.

"The sleeping giant is awake -- wide awake -- and we're paying close attention," said Jaime Contreras, president of the National Capital Immigration Coalition, one of the groups involved in organizing the demonstrations.

The protests began in the morning in communities on the East Coast, then spread across the country throughout the day and continued into the evening.

Two of the largest demonstrations were held in Washington, D.C., and Phoenix, Arizona. Organizers said 500,000 attended the event in Washington and 200,000 in Phoenix, although police did not provide official crowd estimates.

In a nod to criticism of demonstrators who waved foreign flags at earlier pro-immigration rallies, the Stars and Stripes were on prominent display at many of Monday's events.

In Washington, protesters held up signs proclaiming, "We Are America."

"What we want to achieve is to send a very strong message to the Senate, to the Congress in general and this administration that immigrants are fed up, that we are tired, that we work very hard," Contreras said.

"We come to this country not to take from America, but to make America strong. And we do not deserve to be treated the way we have been treated," he said.