By Matt Wells BBC correspondent in New York

America's birthday this year is a balance between celebration and barely suppressed fear.

One small example stands out in my mind more powerfully than the image of 2,000 National Guard troops patrolling New York today, or the six NYPD helicopters which are circling the skies.

I'm not going to let my life be run by fear

Vince Paule, New York visitor

Every hour of the day for weeks now, there has been a lone police officer seated at one end of the platform of my subway station.

It is the last stop on the line in Brooklyn before you reach New York's heartland - Manhattan.

Intrigued, I asked one of the officers why he was there. His reply was chilling:

"I'm guarding the tunnel. In case they try to get in, and blow it up," he said.

Protection with freedom

Imagine the sense of responsibility and vigilance involved.

Which New York beat cop a year ago would have believed that on Fourth of July 2002, they would be guarding non-descript subway tunnels into Manhattan from a terrorist attack?

The evenings firework show is expected to lure thousands

This example also highlights the delicate balancing act that the authorities have been carrying out.

New Yorkers want to know that they are protected as far as possible from another attack on this symbolic day, but there still has to be freedom to celebrate - freedom to be normal.

Mayor Michael Bloomberg summed it up earlier saying, "the more people on the streets, the more you show the terrorists that they can't win".

Show of solidarity

But the celebrations are missing the usual carefree exuberance of yesteryear. The mood is one of angry defiance.

Many natives have left the city, using the raging heat here as another excuse to head for the countryside. Thousands though, have come in from out of town as an act of solidarity.

We have done a lot of flag-waving since 11 September, and patriotism has been so on the surface... the Fourth of July this year was always going to be different because of that

Henry Alford, Manhattan resident

Vince Paule, visiting from California, thought about taking the train to Boston to escape, but decided to stay.

"I'm not going to let my life be run by fear," he said.

But he was not wholly convinced by the heightened police presence either. "It's just providing us with the perception that we're safe," he added.

Across the water from Manhattan in Brooklyn, more residents have decided to stay - feeling perhaps that they will be safer.

Historian Jenny Weisberg, thinks the sombre mood is only natural:

"We can't really use the word celebration today. It's almost as if this is just another post 11 September day like any other. The extra security just heightens the tension," he said.

Checking procedures

The thousands of New Yorkers who are travelling to the main public event of the evening - the Macy's Fireworks Show on the East River - will have had to pass through 14 security checkpoints to get the best view.

Bags are being searched, and alcohol confiscated.

The real emphasis for most, is on being with loved ones, and making it a day of reflection.

"We have done a lot of flag-waving since 11 September, and patriotism has been so on the surface. The Fourth of July this year was always going to be different because of that," Manhattan-based writer Henry Alford said.

Meanwhile, once Independence Day is over, the local precinct officers who guard the subway tunnels into Manhattan will still be there - watching and waiting, for the attack that everyone hopes will never come.