From Times Square to the Las Vegas Strip to the Rose Parade in Pasadena, Calif., police have rolled out unprecedented security measures triggered by a hike in the national terrorism alert to orange, its second-highest level.
In New York, workers sealed manhole covers and removed mailboxes to guard against any potential bomb attack in Times Square. More than 750,000 revelers are expected to gather under the guard of counter-sniper teams and seven police helicopters.
New York City mayor Michael Bloomberg says sharpshooters will be on rooftops and police on the ground will be screening all revelers with metal detectors.
"We're trying to make sure that if we make a mistake - we're making a mistake on the side of being overly cautious, rather than being under-cautious," says Bloomberg.
Armed helicopters are also prowling the Las Vegas Strip, where 300,000 people are expected.
Nevada Gov. Kenny Guinn says security there includes a ban on all non-commercial flights - even though authorities know of no specific threats involving aviation.
Guinn says the precautions may send Vegas partygoers "a mixed message, but I think it is one of comfort to let them know we are doing everything we can."
"I think the level of security this time around within the United States is absolutely unprecedented," said Homeland Security Secretary Tom Ridge, in a CBS News interview.
New York police are focusing more heavily than last year on hotels, landmarks and ferry terminals as a result of their analysis of anti-American "chatter" culled from the Internet and other sources, police Commissioner Ray Kelly said.
"We think it's prudent for us to do that," he said.
Partygoers headed to New York have been warned to expect long delays at bridges and tunnels. Every vehicle on affected routes is subject to a random stop and search, New Jersey officials said. State troopers will be riding the rails to assist transit police on trains going in and out of New York.
Officials nationwide say there are no specific threats to traditional gatherings and they are urging people to go forward with celebrations.
Still, FBI and other federal agents have been sweeping the Las Vegas Strip for weapons or threats, said Ellen Knowlton, FBI special agent in charge in Las Vegas.
The Strip has 74,344 hotel rooms and 18 of the nation's 20 largest hotels.
Nearly the entire Las Vegas police force of about 2,000 officers was to be on duty, plus about 600 jail officers. Authorities also were relying on help from about 4,000 hotel security guards.
On Monday, the U.S. Department of Homeland Security and the Transportation Security Administration said airspace over the Strip would be restricted.
Thousands of local officers and federal agents were to fan out through Pasadena, Calif., where revelers gather along the 5½-mile Rose Parade route and attend the Rose Bowl football game nearby after the parade. Video surveillance cameras were to capture images of spectators lining the streets.
Flights over the Rose Bowl were to be limited to police and military aircraft, and everyone working in the stadium, from hot dog vendors to television camera crew members, must wear photo ID.
Still, terrorism concerns did not dim the enjoyment of visitors to Southern California on Tuesday.
"We decided not to live our lives in fear, and do what we want to do," said Janet Powles, 60, of Rapid City, S.D., as she watched volunteers apply flower petals to floats in the Rose Palace.
The increase in the alert level to orange and reports of threats to Las Vegas did appear to affect business there, however. Deutsche Bank analyst Marc Falcone said New Year's Eve cancellations jumped in the last week, reaching twice the number recorded last year.
Boston is expecting more than a million visitors for its "First Night" arts festival, the nation's oldest such celebration. Security there is to remain consistent with the last two years' events.
Metal trash cans are removed from the Boylston Street parade route and replaced with cardboard boxes, to minimize damage if there is an explosion. And the Massachusetts Bay Transportation Authority was posting security officers and bomb-sniffing dogs on buses, subways and trains carrying revelers into the city.
In smaller cities and rural areas, officials described increased security at public places such as malls and heightened surveillance at vital infrastructure such as bridges, power plants, water systems, airports and ports.