BOSTON, Massachusetts (AFP) - Republicans won a stunning upset in Massachusetts' Senate election, US television networks reported, dealing a potentially fatal blow to President Barack Obama's health care reforms.

Republican Scott Brown pulled off the surprise victory in the historically Democratic state in a rebuke of Obama one year after he took office.

With more than 80 percent of precincts reporting, Brown was ahead by five percentage points over his Democratic rival Martha Coakley, CNN and MSNBC reported.

The lead meant it was impossible for Coakley to make up in the remaining ballots to be counted, US media reported.

BOSTON - In an epic upset in liberal Massachusetts, Republican Scott Brown rode a wave of voter anger to win the U.S. Senate seat held by the late Edward M. Kennedy for nearly half a century, leaving President Barack Obama's health care overhaul in doubt and marring the end of his first year in office.

The loss by the once-favored Democrat Martha Coakley in the Democratic stronghold was a stunning embarrassment for the White House after Obama rushed to Boston on Sunday to try to save the foundering candidate. Her defeat on Tuesday signaled big political problems for the president's party this fall when House, Senate and gubernatorial candidates are on the ballot nationwide.

"I have no interest in sugarcoating what happened in Massachusetts," said Sen. Robert Menendez, the head of the Senate Democrats' campaign committee. "There is a lot of anxiety in the country right now. Americans are understandably impatient."

Brown will become the 41st Republican in the 100-member Senate, which could allow the GOP to block the president's health care legislation and the rest of his agenda. Democrats needed Coakley to win for a 60th vote to thwart Republican filibusters.

The Republican will finish Kennedy's unexpired term, facing re-election in 2012.

Brown led by 52 per cent to 47 percent with all but 3 percent of precincts counted.

One day shy of the first anniversary of Obama's swearing-in, the election played out amid a backdrop of animosity and resentment from voters over persistently high unemployment, Wall Street bailouts, exploding federal budget deficits and partisan wrangling over health care.

For weeks considered a long shot, Brown seized on voter discontent to overtake Coakley in the campaign's final stretch. His candidacy energized Republicans, including backers of the "tea party" protest movement, while attracting disappointed Democrats and independents uneasy with where they felt the nation was heading.

A cornerstone of Brown's campaign was his promise to vote against the health care plan.