NEW YORK -- As we head into the holiday weekend, America is putting on a display of patriotism -- and vigilance.
As pyrotechnicians were getting the fireworks ready to celebrate the country's 239th birthday, law enforcement was on guard against any attempts to crash the party.
New York City is deploying 7,000 police officers this weekend. Some will be heavily armed, standing watch along with K9 units.
Others will patrol the Hudson River on boats capable of detecting radiation. More officers will watch from overhead in specially-equipped helicopters.
"The helicopters have many capabilities. They are able to do an aerial survey with radiological detection equipment," James Waters, chief of the NYPD's counterterrorism unit, tells CBS News. "They also have cameras that they are able to see basically the dimple on someone's chin from 1,300 or 1,400 feet in the air."
At the NYPD's command center, officers will keep a close eye on thousands of cameras.
Security is being tightened across the country. In Washington, some 700,000 people are expected on the National Mall, and police will have special patrols around major landmarks.
In Chicago, officers will be working 12-hour shifts to help the city handle its fireworks show, Grateful Dead concerts, and baseball games.
And in Boston, law enforcement is keeping a close eye along the Charles River, where hundreds of thousands of people will gather for the Boston Pops fireworks show.
While police are on a heightened state of alert, they are encouraging the public to enjoy the holiday.
"My message, quite frankly, to anyone that is intent on causing us harm here in this city: We are well prepared and we're ready," Waters says.
Across the country, the FBI has set up so-called "fusion centers" to share information and coordinate response. Police in New York City and elsewhere emphasize, if you see something suspicious, say something to authorities.