"I am pleased to announce, that with your help, the next vice president of the United States will be Sen. John Edwards of North Carolina," Kerry told a boisterous rally in downtown Pittsburgh.
It was the first time a candidate used the Internet for such a crucial announcement. A campaign aide said about 150,000 people had signed up since Kerry told reporters on Friday they would hear the news first.
The presumptiveto face in November, Kerry credited Edwards, a first-term senator and millionaire trial lawyer, with "guts, determination and political skills."
"He has shown courage and conviction as a champion for middle class Americans and those struggling to reach the middle class," Kerry said of Edwards, who had been Kerry's chief rival in the party's primary selection process.
Many Democrats were impressed with his charismatic style and populist message in his spirited campaign against Kerry for the party's presidential nod. Edwards has since raised money for Kerry and had lobbied openly for No. 2 spot. While he was not at Kerry's side when the announcement was made, the two were to campaign together for the next few days.
The two men will be formally anointed later this month at the Democratic convention in Boston as the party's challengers to Republicans Bush and Vice President Dick Cheney in the Nov. 2 election.
Kerry said he was excited at the prospect of running with Edwards, his colleague from the Senate and a rival during the Democratic presidential primaries, and seeing him square off in debate this fall with Cheney.
"I can't tell you how proud I am to have John Edwards on my team, or how eager I am for the day this fall when he stands up for our vision and goes toe-to-toe with Dick Cheney," Kerry said.
Edwards, 51, won praise for a strong populist campaign that stressed proposals aimed at helping working class Americans and made repeated references to the "two Americas"--one for the wealthy and one for those struggling to get by.
But Republicans pounced on the one-term senator's relative inexperience and lack of national security credentials with the United States at war against terrorism.
The Bush campaign immediately announced the release of an new television advertisement featuring Kerry's "first choice" as running mate, Republican Sen. John McCain of Arizona. The Democrat courted his colleague and fellow Vietnam veteran for the job but was rebuffed.
Kerry, a senator from Massachusetts, conducted the vice presidential search under extraordinary secrecy and stayed mum during an elegant barbecue for fellow Democrats at his wife's $3.7 million family farm on Monday.
"I'm going to keep this process very personal and private...until the moment I make an announcement," he told local television stations.
Despite the secretive selection process, complete with clandestine meetings and limited to just a few close aides, several names, including those of Edwards, Missouri Rep. Richard Gephardt, Florida Sen. Bob Graham and Iowa Gov. Tom Vilsack had percolated to the top.
Kerry had dropped no hints about his thinking to 350 guests dining on hamburgers and roaming among hay bales and balloons on the hilltop lawns of Teresa Heinz Kerry's 90-acre "Rosemont Farm" outside Pittsburgh.
But he did urge Democrats to turn out for the rally, billed as the start of a three-day focus on "restoring responsible leadership" to the White House.
"We're going to have some fun," Kerry said.
The rollout of a running mate and the party conventions usually provide a candidate with big surges in the polls. Democratic National Committee Chairman Terry McAuliffe predicted Kerry would get a bump of between 8 and 12 percentage points.