Democrats Barack Obama and Hillary Clinton traded early Super Tuesday victories but nervously awaited results in key battlegrounds after the biggest one-day White House nominating showdown in US history.
Republican John McCain meanwhile swept to an early lead as he hoped to secure his party's nomination in the 24-state clash, grabbing wins in Illinois, Connecticut and New Jersey.
But the 71-year-old Vietnam war hero was locked in a tight struggle in Georgia with Mike Huckabee, who won West Virginia, showing McCain's weakness with the key conservative bloc.
Another key rival, Mitt Romney, hoping to also corral conservatives against McCain hung on to his home state of Massachusetts.
Obama, a charismatic senator, vying to become the first black president in US history, won Georgia and most of its 87 delegates, where he got 86 per cent of African American voters, according to television network projections.
He also, as expected, won his home state of Illinois.
Polls had closed in closely watched Democratic races in Connecticut, Massachusetts, New Jersey and Missouri, states where Clinton had enjoyed wide leads a few weeks ago but had to battle an Obama surge in recent days.
The former first lady, hoping to get back to the White House as America's first woman president meanwhile won Oklahoma and southern Tennessee, one of the must-win states on her road to the Democratic nomination.
As expected, Clinton and Huckabee had wins in Arkansas, where her husband was once governor as was Huckabee.
Turnout was reportedly high as voting rippled across the country from northeastern Massachusetts to western Alaska, in a 24-state bonanza of primaries and caucuses, the biggest test yet in the enthralling 2008 race.
Clinton campaign aides said they expected the night's results to be inconclusive, setting up a prolonged struggle which could drag on for several months. The Obama campaign hoped to fight to a rough draw.
The full picture of Super Tuesday voting will only emerge in California, where polls close at 0400 GMT, where both the Democratic and Republican races are too close to call.
Senator Clinton, 60, voice husky from fatigue, has vowed to fight on as opinion surveys picked up a surge by Obama, 46.
"There's never been a day like this," Clinton told MSNBC after voting in her hometown of Chappaqua, New York.
Obama, speaking to NBC television, said it was likely the battle would go on for weeks to come before a Democratic presidential nominee for the November elections was decided.
"I suspect that we're going to probably see a split decision tonight and then we're going to have time over the next month, month and a half, to continue the campaign," he said.
After a clutch of single-state contests, Super Tuesday embraces millions of voters from across racial, religious, social and income barriers, in states as diverse as liberal Massachusetts and parched Arizona in the southwest.
The first voter to emerge from one New York City polling station said he had voted for Obama.
"For so many reasons. I think Hillary has so much baggage, I want a black president whose middle name is Hussein and he seems like a great guy," said Stuart Bernstein, 47, a literary agent.
But two Hispanic women said they had plumped for Clinton, whom they saw as a safe pair of hands and a friend of Latino and other immigrant groups.
"She has experience. We like her. She can be a good president," said Elena Zingaretti, a 66-year-old Colombian-born domestic worker.
Clinton went into the clash after pocketing wins in contests in New Hampshire and Nevada, while Obama took the leadoff Iowa caucuses and thumped her in the South Carolina primary.
Super Tuesday states allocate more than half the Democratic delegates and almost half of Republican delegates to the party conventions in August and September, which will formally nominate candidates for November's general election.
There are Democratic contests in 22 states, Republican match-ups in 21 states, and 19 states are holding nominating clashes for both parties.
US Democrats abroad were also getting their first chance to vote in Super Tuesday primaries in polling stations set up in clubs, bars and even churches in capitals as far flung as London to Jakarta.
Early Tuesday, McCain said he did not want to jinx his recent run of good luck, after his campaign was all but given up for dead last year.
But he said on his plane: "I'm always nervous and always superstitious. And I'm always a pain in a certain part of the anatomy to most of my friends and associates." (AFP)
Obama, Clinton await key results in tense Super Tuesday