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By Alex Johnson

"I think we'll be OK."

With those words, singer Taylor Hicks summed up the message authorities were hoping to send as millions of Americans celebrated Independence Day under the watchful eyes of noticeably beefed-up security details across the country.

Hicks was in Las Vegas for the holiday weekend, helping Caesar's Palace celebrate its 50th anniversary, NBC station KSNV reported. Like most U.S. towns and cities, Las Vegas deployed maximum visible security at airports and Fourth of July events to reassure jittery Americans.

U.S. officials told NBC News that intelligence reports didn't indicate any specific threat or imminent attack for the long holiday weekend. Still, in the wake of the mass shooting in Orlando, Florida, and terrorist attacks in Baghdad, Iraq, and Istanbul, Turkey, and across Saudi Arabia, officials weren't taking any chances.

ISIS claimed responsibility for the Iraqi attacks, and U.S. officials have told NBC News that the Turkish and Saudi attacks bore the hallmarks of ISIS operations.

Related: After Istanbul Attack, Security Stepped Up at U.S. Airports for Holiday

Police and private security swarmed San Francisco's Pier 39 a half-hour before Monday night's fireworks display, protecting pyrotechnic workers from any potential interference, NBC Bay Area reported.

"We have higher security at the loading site," said John Erickson, a pyrotechnic operator. "The Coast Guard will be around, San Francisco police will have their launches out, keeping boats away from us."

In New York City, officials debuted a critical response command staffed with hundreds of officers, both in and out of uniform.

"We believe this is the largest detail in modern times for this event," Police Commissioner William Bratton said Monday.

A New York police officer and the Statue of Liberty exchange notes Monday in Times Square.NBC News

The tight security is "reflective of the changing world conditions, which we've been watching closely this past week and indeed this past year," Bratton said.

And "there's also a lot you won't see," New York Police Chief James O'Neill said, noting that many of the officers were in plainclothes or were working from centralized high-tech centers monitoring cameras covering almost every inch of the city.

Long lines snaked from security checkpoints to enter Boston's Esplanade, where the Boston Pops, Nick Jonas, Demi Lovato and the country group Little Big Town were scheduled to perform Monday night before the city's annual fireworks show.

"We're certainly on a heightened sense of vigilance out here," Massachusetts State Police Sgt. Tom Ryan told NBC station WHDH.

In Atlanta, similar measures have been the norm for several years.

Atlanta is home to one of the country's largest 10-kilometer races, the Peachtree Road Race, which is held every Independence Day. And ever since two brothers killed three people and injured more than 250 others at the 2013 Boston Marathon, Atlanta police have vowed to keep their race safe.

"We've spent countless hours going over a number of scenarios, ranging from bombings to extreme heat," Police Chief George Turner told reporters.

Police line Constitution Avenue along the route of the Independence Day parade Monday in Washington, D.C.Michael Reynolds / EPA

Police snipers lined the race route and the sites of fireworks displays Monday. National Guard troops and undercover officers mingled among the crowds. And surveillance cameras were everywhere to catch anything that human eyes might miss, NBC station WXIA reported.

"The way that we began to prepare for this event changed after Boston," Turner said. "We're all prepared to try to mitigate any situation."

It's not just big cities that were on special alert. In Bangor, Maine, police said they've been especially vigilant about planning for Independence Day ever since the Orlando shootings last month, NBC station WCSH of Portland reported.

Police called in extra officers to make sure Bangor's road race, concert and fireworks show were all fully staffed, along with the annual July 4 parade.

"The parade — you just never know what to expect," said Michael Fern, president of the Greater Bangor 4th of July Corp.

The message from authorities across the country was summed up by daredevil Robbie Knievel, who was on hand at the Caesar's Palace anniversary celebration Sunday night in Las Vegas.

Knievel surveyed the squads of bomb-sniffing dogs and uniformed cops who'd turned out in droves, KSNV reported, and he needed just two words to get the point across:

"Screw ISIS."

People watch the Independence Day parade behind a police officer Monday in Washington, D.C.Michael Reynolds / EPA