The race was seen as a harbinger of sentiment over a conflict that has claimed the lives of more than 2,500 US troops.
Unbowed, Lieberman vowed to fight on, announcing plans to enter the fall campaign as an independent this fall. Only six years ago, Lieberman was the Democrats' choice for vice president.
"Of course I am disappointed by the results, but I am not discouraged," Lieberman said. "For the sake of our state, our country and my party, I cannot and will not let that result stand."
Lamont won with 52 percent of the vote, or 146,061, to 48 percent for Lieberman, with 136,042, with 99 percent of precincts reporting. Turnout was projected at twice the norm for a primary.
"As I see it, in this campaign we just finished the first half and the Lamont team is ahead. But, in the second half, our team, Team Connecticut, is going to surge forward to victory in November," Lieberman said after congratulating Lamont.
In Tuesday's primary elections, voters were choosing their party's candidates for the November general election.
Lamont, a millionaire with virtually no political experience, ran on his opposition to the Iraq war.
"They call Connecticut the land of steady habits," a jubilant Lamont told cheering supporters. "Tonight we voted for a big change."
Democratic critics targeted Lieberman for his strong support for the Iraq war and for his close ties to President George W. Bush. They played and replayed video of the kiss President Bush planted on Lieberman's cheek after the 2005 State of the Union address.
Lieberman's loss made him only the fourth incumbent US senator to lose a primary since 1980.
Turnout was projected at twice the norm for a primary.
In Georgia, US Rep. Cynthia McKinney, the fiery congresswoman known for her conspiracy theories about the Sept. 11 attacks and a scuffle this year with a US Capitol police officer, lost a runoff for the Democratic nomination.
And in Michigan, moderate Republican Rep. Joe Schwarz lost to a conservative in a Republican primary.
Elsewhere, voters in Colorado and Missouri also chose candidates for the November midterm elections when control of both houses of the US Congress will be at stake.
The Connecticut Senate race dominated the political landscape in recent weeks, as Lamont demonstrated the power of anti-war sentiment among Democrats with his campaign. Lamont is the millionaire owner of a cable television company, but his political career is limited to serving as a town councilman and member of the town tax board.
It was a race watched closely by the liberal, Internet-savvy Democrats who lead the party's emerging "netroots" movement, groups such as Moveon.org that played a big role in pushing Lamont's candidacy.
Officials said turnout was up to 50 percent when primaries usually only draw 25 percent of voters. And vote totals showed roughly 16,000 more ballots cast for the Democratic Senate primary than the party primary for governor, reflecting the extra attention to the Lieberman-Lamont battle.
Jubilant Lamont supporters predicted victory in November.
"People are going to look back and say the Bush years started to end in Connecticut," said Avi Green, a volunteer from Boston. "The Republicans are going to look at tonight and realize there's blood in the water."
On the final day of the race, Lieberman accused his opponent's supporters of hacking his campaign Web site and e-mail system. Campaign manager Sean Smith said the site began having problems Monday night and crashed for good at 7 a.m., denying voters information about the candidate.
"It is a deliberate attempt to disenfranchise voters," Smith said.
Lamont, campaigning early Tuesday afternoon in Bridgeport, said he knew nothing about the accusations. "It's just another scurrilous charge," he said.
In the lead up to Tuesday's primary, 14,000 new Connecticut voters registered as Democrats, while another 14,000 state voters switched their registration from unaffiliated to Democrat to vote in the primary.
In Georgia, McKinney, her state's first black congresswoman, lost to Hank Johnson, the black former commissioner of DeKalb County, 58 percent to 41 percent.
In the heavily Democratic district, the runoff winner is likely to win in the fall.
McKinney has long been controversial, once suggesting the Bush administration had advance knowledge of the Sept. 11, 2001, terror attacks. Her comments helped galvanize opposition and she lost her seat in 2002, but won it again two years ago.
In her latest controversy in March, she struck a Capitol Police officer who did not recognize her and tried to stop her from entering a House office building.
A grand jury in Washington declined to indict her, but she was forced to apologize before the House. She drew less than 50 percent of the vote in last month's primary.
In other primaries:
— In Michigan, Schwarz, a moderate Republican congressman who supports abortion rights, lost to conservative Tim Walberg, a former state lawmaker. The race has drawn more than $1 million (¤780,000) from outside groups; Schwarz has received support from President Bush and Arizona Sen. John McCain.
— In Colorado, two open congressional seats have drawn crowds of candidates.
— In Missouri, Republican US Sen. Jim Talent and Democratic challenger Claire McCaskill, the state auditor, won their party's primaries.
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