Spectators waving US flags on Thursday and wearing red, white and blue headed for public gatherings in Boston, New York, Washington, Atlanta and other cities under the close watch of police armed with hand-held chemical detectors, radiation scanners and camera surveillance – precautions sparked by the deadly April 15 bombings.
A US national security official said on Wednesday that US intelligence agencies were unaware of any attack threat by militants timed to occur on July 4.
Under steamy summer skies, tourists in New York flocked to ferries headed for the Statue of Liberty, re-opening with an Independence Day ceremony after closing in October as Superstorm Sandy approached.
About 75% of Liberty Island, the statue's home off the southern tip of Manhattan, was swamped by surges during the powerful storm.
Others headed for Brooklyn's Coney Island to cheer on competitors in the annual Nathan's Famous hot dog eating contest.
Six-time champion Joey "Jaws" Chestnut will strive to break his own world record of 68 hot dogs and bread rolls in 10 minutes and hold onto the Mustard Yellow International Belt. His female counterpart, 48kg Sonya "The Black Widow" Thomas, seeks to defend her record of 45 hot dogs and bread rolls.
Meanwhile, former men's champ Takeru Kobayashi will be swallowing hot dogs at a separate contest in Manhattan. He will try to break his personal record of 69 dogs in 10 minutes – against six other competitive eaters.
Kobayashi has been banned from the Coney Island match for refusing to sign an exclusive contract with the organisers, Major League Eating. He rushed the stage three years ago, wearing a "Free Kobi" T-shirt, and was taken away by police.
Amid controversy over revelations about the National Security Agency (NSA), some marked the nation's 237th birthday by demanding respect for its founding principles.
In Birmingham, Alabama, organisers are holding a "Restore the Fourth" protest – a reference to the US Constitution's Fourth Amendment protections against unreasonable searches and seizures – to denounce NSA surveillance programmes.
The NSA scandal weighed heavily on the minds of some parade-goers in Maplewood, New Jersey.
"I think we've lost our way," said Joe Kyle (44), who teaches US history at a public high school in New Jersey and was pulling his two young daughters in a red wagon festooned with a half-dozen American flags.
Referring to Edward Snowden, a former NSA contractor wanted by Washington for leaking secrets about government surveillance, Kyle said: "I think he should be celebrated, not prosecuted."
His wife Kate Ritchie (34), who is an attorney, said Snowden should be criminally charged, although she was not surprised by his allegations of NSA spying.
"I don't like it but it's not a shocking revelation," Ritchie said. "There is still plenty about our country to celebrate at this time."
July 4 celebrations mark one of the largest public gatherings since bombs exploded at the finish line of the Boston Marathon, the biggest attack on American soil since the September 11 2001, attacks on New York's World Trade Centre towers and the Pentagon.
Tamerlan and Dzhokhar Tsarnaev, the two brothers suspected of carrying out the attack on the marathon, originally planned to set off their homemade bombs on July 4 but struck earlier because they had made the devices sooner than expected, law enforcement officials have said.
Tamerlan was killed in a shootout with police and Dzhokhar is in prison awaiting trial on charges including murder and using a weapon of mass destruction. He could face the death penalty if convicted. – Reuters