BOSTON — Scott Brown, a little-known Republican state senator, rode an old pickup truck and a growing sense of unease among independent voters to an extraordinary upset Tuesday night when he was elected to fill the Senate seat that was long held by Edward M. Kennedy in the overwhelmingly Democratic state of Massachusetts.

By a decisive margin, Mr. Brown defeated Martha Coakley, the state’s attorney general, who had been considered a prohibitive favorite to win just over a month ago after she easily won the Democratic primary.

Brian Snyder/Reuters

Scott Brown celebrated with his wife Gail Huff and daughters after defeating Democrat Martha Coakley in the U.S. Senate race on Tuesday night. More Photos >

With all precincts counted, Mr. Brown had 52 percent of the vote to Ms. Coakley’s 47 percent.

“Tonight the independent voice of Massachusetts has spoken,” Mr. Brown told his cheering supporters in a victory speech, standing in front of a backdrop that said “The People’s Seat.”

The election left Democrats in Congress scrambling to salvage a bill overhauling the nation’s health care system, which the late Mr. Kennedy had called “the cause of my life.” Mr. Brown has vowed to oppose the bill, and once he takes office the Democrats will no longer control the 60 votes in the Senate needed to overcome filibusters.

There were immediate signs that the bill had become imperiled. House members indicated they would not quickly pass the bill the Senate approved last month.

And after the results were announced, one centrist Democratic senator, Jim Webb of Virginia, called on Senate leaders to suspend any votes on the Democrats’ health care legislation until Mr. Brown is sworn into office. The election, he said, was a referendum on both health care and the integrity of the government process.

Beyond the bill, the election of a man supported by the Tea Party movement also represented an unexpected reproach by many voters to President Obama after his first year in office, and struck fear into the hearts of Democratic lawmakers, who are already worried about their prospects in the midterm elections later this year.

Mr. Brown was able to appeal to independents who were anxious about the economy and concerned about the direction taken by Democrats, now that they control both Beacon Hill and Washington. He rallied his supporters when he said, at the last debate, that he was not running for Mr. Kennedy’s seat but for “the people’s seat.”

That seat, held for nearly half a century by Mr. Kennedy, the liberal lion of the Senate, will now be held for the next two years by a Republican who has said he supports waterboarding as an interrogation technique for terrorism suspects, opposes a federal cap-and-trade program to reduce carbon emissions and opposes a path to citizenship for illegal immigrants unless they leave the country. It was a sharp swing of the pendulum, but even Democratic voters said they wanted the Obama administration to change direction.

“I’m hoping that it gives a message to the country,” said Marlene Connolly, 73, of North Andover, a lifelong Democrat who said she cast her first vote for a Republican on Tuesday. “I think if Massachusetts puts Brown in, it’s a message of ‘that’s enough.’ Let’s stop the giveaways and let’s get jobs going.”

Mr. Brown ran strongest in the suburbs of Boston, where the independent voters who make up a majority in Massachusetts turned out in large numbers. Ms. Coakley did best in urban areas, winning overwhelmingly in Boston and running ahead in Springfield, Worcester, Fall River and New Bedford, but her margins were not large enough to carry her to victory.

In a concession speech before cheering supporters, Ms. Coakley acknowledged that voters were angry and said she had hoped to deal with the concerns.

“Our mission continues, and our work goes on,” she said, echoing well-known remarks by Mr. Kennedy. “I am heartbroken at the result, as I know you are, and I know we will get up together tomorrow and continue this fight, even with this result tonight.”

The crowd at Mr. Brown’s victory rally, upset by reports that Democrats might try to vote on the health care bill before he takes office, chanted, “Seat him now!” Mr. Brown, for his part, noted that the interim senator holding the seat had finished his work, and that he was ready to go to Washington “without delay.” And he effusively praised Mr. Kennedy as a big-hearted, tireless worker, and said that he hoped to prove a worthy successor to him.

Ms. Coakley’s defeat, in a state that Mr. Obama won in 2008 with 62 percent of the vote, led to a round of finger-pointing among Democrats. Some criticized her tendency for gaffes — in a radio interview she offended Red Sox fans when she incorrectly suggested that Curt Schilling, a beloved former Red Sox pitcher, was a Yankee fan — while others criticized a lackluster, low-key campaign.

Mr. Brown presented himself as a Massachusetts Everyman, featuring the pickup truck he drives around the state in his speeches and one of his television commercials, calling in to talk radio shows and campaigning with popular local sports figures.

The implications of the election drew nationwide attention, and millions of dollars of outside spending, to the race. It transformed what many had expected to be a sleepy, low-turnout special election on a snowy day in January into a high-profile contest that appeared to draw more voters than expected to the polls. There were reports of traffic jams outside suburban polling stations, while other polling stations had to call for extra ballots.

The late surge by Mr. Brown appeared to catch Democrats by surprise, causing them to scramble in the last week and a half of the campaign and hastily schedule an appearance by Mr. Obama with Ms. Coakley on Sunday afternoon.

“Understand what’s at stake here, Massachusetts,” Mr. Obama said in his speech that day, repeatedly invoking Mr. Kennedy’s legacy. “It’s whether we’re going forwards or backwards.” He all but pleaded with voters to support Ms. Coakley, to preserve his agenda.

As voters went to the polls, Robert Gibbs, the White House press secretary, made it clear that the president was “not pleased” with the situation Ms. Coakley found herself in. “He was both surprised and frustrated,” Mr. Gibbs said.

Although the race has riveted the nation largely because it was seen as contributing to the success or defeat of the health care bill, the potency of the issue for voters here was difficult to gauge. That is because Massachusetts already has near-universal health coverage, thanks to a law passed when Mitt Romney, a Republican, was governor.

Thus Massachusetts is one of the few states where the benefits promised by the national bill were expected to have little effect on how many of its residents got coverage, making it an unlikely place for a referendum on the health care bill.

On Capitol Hill, the fate of the health care legislation was highly uncertain as Democratic leaders quickly gathered to plot strategy in the wake of the Republican victory.

Sentiment about how to proceed was mixed, with several lawmakers saying the House would not accept the Senate-passed plan. Top officials had said that approach was the party’s best alternative, and many members said they still believed it was crucial that Democrats pass a plan.

“It is important for us to pass legislation,” said Representative Baron P. Hill, a conservative Democrat from Indiana.