EW ORLEANS, Feb. 3 — Adam Vinatieri's 48-yard field goal as time expired lifted the New England Patriots to the most dramatic finish in Super Bowl history tonight, a stunning, magical 20-17 upset victory over the St. Louis Rams in Super Bowl XXXVI before 72,922 fans at the Superdome.

In a city known for its reveling ways, this game and that kick rocked pro football. The Patriots felt they had been given little chance to win this game, but they played solidly on offense, competently on special teams and excelled on defense, shutting down a potent Rams offense.

Prolific? Stylish? Not tonight. Through three quarters the Rams managed 3 points. It took two touchdowns in the final 10 minutes, the last one with 1 minute 37 seconds left, for St. Louis to tie the score at 17-17.

It was the New England offense, however, that gained the last yards and the last laugh, driving from its 17-yard line to the St. Louis 30 to set up Vinatieri's kick. The key play was Tom Brady's pass of 23 yards to Troy Brown to the St. Louis 36.

This Super Bowl was unlike any other in terms of the depth of security. Eleven law enforcement agencies, led by the Secret Service, participated. The extra security was evident outside and inside the Superdome and even in the air, where helicopters and fighter planes patrolled. Fans — who were patted down from head to toe before entering the stadium — anticipated the extra security and arrived early.

Those fans were treated to a pregame concert that focused on freedom. Paul McCartney, Patti LaBelle, Marc Anthony, Mary J. Blige and Barry Manilow were among the performers. The national anthem was sung by Mariah Carey, and the fans in the three decks of the Superdome, from top to bottom, waved lights that were red, white and blue.

Once the game began, the Patriots quickly strutted their stuff. They jumped on top, by 14-3 at halftime, scoring twice off turnovers, and by 17-3 early in the the fourth quarter.

Bill Belichick, the Patriots' coach, said: "We had to rush them with everything but the kitchen sink. We had to alternate our coverages. We disrupted them. Our players believe in themselves. They beat the No. 1 seed in the A.F.C. in Pittsburgh to get here, and now they have beaten the No. 1 seed in the N.F.C. in St. Louis."

Kurt Warner, the Rams' quarterback, struggled all game long. "To get this far and not finish it, that hurts," said Warner, who was 28 of 44 for 365 yards. "I made some mistakes today that cost us. They played aggressively but we did, too. They played a better game."

Warner's counterpart, Tom Brady, a 24-year-old second-year player who entered the season as a backup, ended it as the youngest quarterback to win a Super Bowl. He was named its most valuable player after completing 16 of 27 passes for 145 yards and a touchdown and managing the game superbly for the Patriots.

Brady won a Cadillac for the M.V.P. award, but the team he drove was more like a van. Emblematic of that were the introductions before the game. The Rams chose their offense to be introduced, but the Patriots chose to introduce neither their offense or defense but their entire team as one unit. How prophetic.

"Everybody got it done," Brady said. "We were the true reflection of team out there today."

The Rams scored first on Jeff Wilkins's 50-yard field goal (third longest in Super Bowl history), but in the second quarter the Patriots began a string of 17 consecutive points.

The first came on Ty Law's 47-yard interception return for a touchdown with 8:49 left before halftime.

On first-and-10 at the St. Louis 39, Warner dropped back to pass. Linebacker Mike Vrabel was free to blitz cleanly off the left corner and, before pulverizing Warner in the pocket, he got a hand in Warner's face, obscuring his vision. Warner flung an out pass toward the sideline, where Isaac Bruce was running an out pattern. But the ball sailed wide and long and into Law's hands.

"That was the momentum right there," Law said. "That started it off. We rode it from there. It was a great opportunity. No one thought we could run with these guys, but we proved them wrong again today. That is the best group of receivers in a long time, but we were the better team today."

The Rams had more yards than the Patriots (427 to 267), ran more plays (69 to 54) and held the ball seven minutes longer. But turnovers can turn a game upside down.

"They did a good job, obviously," Rams Coach Mike Martz said. "We had a couple of protection breakdowns. Obviously you can't make those kind of errors against a team like this. I think if we don't turn the ball over we're in pretty good shape. But to say that insults this Patriots team. They won this championship."

Warner's interception preceded another Rams turnover, with 1:20 left before halftime, when receiver Ricky Proehl put the ball on the turf. Proehl made a 15-yard catch to the St. Louis 30, where safety Antwan Harris put his helmet on the ball. Out it squirted, into Terrell Buckley's hands. His 15-yard return put the ball at the St. Louis 40.

And for the first time, the New England offense scored, driving the 40 yards in 49 seconds. The touchdown came on David Patten's leaping grab of an 8-yard pass by Brady. With 31 seconds left, New England had a 14-3 lead.

"This one was supposed to be an out route but Coach Belichick changed it on Friday, the last day of practice, and we tried to run an out and up on them and it worked for us," Patten said. "It was a great throw and I was able to make a good adjustment on the ball. That's the type of play you have to expect out of yourself in a game of this magnitude."

The type of plays the Rams' offense makes. But not tonight.

New England kept making the Rams pay for their mistakes. Warner made another turnover in the third quarter, the Rams' third and final one. Warner had not thrown a interception in his first 60 Super Bowl passes but here came his second in this game, an Otis Smith theft and return of 30 yards to the St. Louis 33. His return led to a Vinatieri field goal of 37 yards and a 17-3 lead.

The Rams had an answer, but only briefly. Warner ran for a 2-yard score with 9:31 remaining and Proehl made a 26-yard scoring catch with 1:30 left.

The Patriots began their game-winning drive with the ball at their 17 line. New England played to win. They made it a 60-minute game, with every second counting. "We wanted to take a couple of shots and see what happened," Belichick said.

And those shots inched the Patriots closer and closer, more than close enough for Vinatieri.

His kick helped lift the Patriots to the first championship in their 42 years and helped erase the bad memories here of the Super Bowl that New England lost to Green Bay in 1997. It booted aside the favored Rams, who two years ago won the Super Bowl on a last-second defensive play at the Rams' 1-yard line against the Tennessee Titans.

"We're capable of coming back from huge deficits," Rams running back Marshall Faulk said. "Before all that, we put ourselves in a huge hole with turnovers. Sometimes we start slow and we can pick it up. For us, it's a four-quarter ball game and we feel we can outrun anybody in any race. Today, we were outrun."

Brady offered the final word: "We've got a whole team full of underdogs and now we're the top dogs."