The FBI has ordered its field offices to keep watch over holiday events because of the danger that terrorists might strike.
With more than 1 million people expected in Philadelphia, authorities will be on high alert, guarding such historic landmarks as Independence Hall. More than 500,000 people are expected in Boston, where access to events will be restricted and bags and coolers will be searched.
In the nation's capital, a double fence will be up around the Mall and the Capitol grounds, with checkpoints for searching people and their bags.
In New York, Mayor Michael R. Bloomberg has lifted a fireworks ban put in place after Sept. 11 for a show called "A Time for Heroes." A lineup of celebrities will narrate, and the event will culminate in a barrage of 20,000 pyrotechnic shells fired from barges in the East River.
"Given what is going on in the world, this will be a very special year for our fireworks, because the true meaning of the celebration will resonate more with people," said Robin Hall, producer of the New York City show. "There is a surge of real patriotism in America, heroism has been redefined for many of us and liberty and freedom have a new immediacy."
Washington, San Francisco and Chicago will remember the victims and the heroes of Sept. 11 with a fireworks salute called The American Tribute.
"There will be a silent sky, and then a solitary spectacular burst of red, followed once the sky has cleared by a solitary spectacular burst of white, followed by a solitary spectacular burst of blue," said Jim Van Eerden, who came up with the idea with his family and wrote about it in a letter to President George W. Bush. The tribute is now planned as part of the Capitol's celebration and dozens of other events across the country.
While many fireworks displays will be more spectacular, parades are expected to be more somber.
In Washington, widows of generals killed on Sept. 11 at the Pentagon will be in the reviewing stands, and Pentagon police will be parade marshals. "It's going to be powerful this year," said producer Pat Wolverton.
Baton Rouge, Louisiana, canceled its mock World War II air-sea battle on the Mississippi River that has been a fixture of holiday observations for 15 years because the warfare would be too much of a reminder of the real thing.