Celebrations marking the nation's 239th birthday faced what some call the most serious threats since 2001.
"The threat stream is very high," Los Angeles Deputy Police Chief Michael Downing said. "In fact, we don't think it's been this high since 9/11."
People attending the fireworks on the National Mall had to pass through one of 11 checkpoints set up to keep visitors safe, and in New York City 7,000 extra officers were deployed for the fireworks. In Chicago, police were put on 12-hour shifts for the holiday.
Authorities have cited no specific plot or threat but are concerned about ISIS and supporters possibly lurking inside the United States.
"Obviously, a lone wolf is our worst nightmare," Boston Police Commissioner William Evans said, "but there's nothing to indicate we have any of that going on."
"You just pray that it doesn't happen," Dan Hagan of Columbia, South Carolina, said.
Added Shola Sutton of Houston: "I've seen police all over the place."
But despite all the security, it's always important to say something if you see something, authorities say, because the best defense is what they call people's "sixth sense."
"If you believe something is wrong or that the hair stands up on the back of your neck, don't feel ashamed or embarrassed to reach out to a member of law enforcement that is in the area to let them know, 'Hey, please check this,'" U.S. Park Police Chief Robert MacLean said.