John McCain has forged ahead in his bid to win the US Republican presidential nomination with primary wins in several big states, according to projections.
For the Democrats, Hillary Clinton won Super Tuesday's biggest prize, California, US media said. However, rival Barack Obama was also doing well.
Meanwhile Republican outsider Mike Huckabee has done better than expected, with five apparent victories.
It is the biggest day yet in the race to choose presidential candidates.
In each state's primary or caucus, successful candidates are awarded delegates who formally choose the party's presidential nominee at the national convention in six months' time.
Overall, 42% of those delegates are being chosen in the 24 states voting on Super Tuesday.
Senator McCain, the Republican front-runner, has seized New York, New Jersey, Missouri, Connecticut, Delaware, and his home state of Arizona - in races where the winner takes all the delegates.
New York alone comes with 87 delegates to the Republican convention, while the others carry 205 between them.
Mr McCain is also projected to have won California, Illinois and Oklahoma.
The Democratic race was as tight as was forecast, although Mrs Clinton is ahead in the two biggest states.
In New York, with almost all the votes counted, she captured about 57% of votes to Mr Obama's 40%.
In California, with about a third of precincts counted, Mrs Clinton had won more than 50% of Democratic votes and Mr Obama about 37%.
But the Illinois senator took a greater number of states, including some in Mrs Clinton's north-eastern backyard.
In terms of delegates captured, Mrs Clinton was only marginally ahead under the Democrats' system of proportional distribution.
Momentum regained
Mr Obama seized the early momentum, taking Georgia and his home state of Illinois - both big states.
"I think we must get used to the idea that we are the Republican Party front-runner... and I don't really mind it one bit"
Georgia, with a large African-American population, was expected to fall to him - but analysts picked up on the racial breakdown in the vote. In South Carolina last month, they noted, only 24% of white voters chose Mr Obama. In Georgia the figure was 43%, suggesting he may have broadened his appeal.
But Hillary Clinton's campaign brushed off the early defeat, saying: "Unlike the Obama campaign, the Clinton campaign never dedicated significant resources to Georgia."
Mrs Clinton then rebounded, with projected wins in Oklahoma, Tennessee and Arkansas - showing, her campaign team said, that she could win in "red", (traditionally Republican) states.
She then went on to take big-prize states.
Meanwhile Mr Obama added Utah, North Dakota, Alabama, Delaware, Connecticut, Kansas, Colorado and Minnesota.
The senator from Illinois told supporters: "We don't need the final results to know our time has come."
Exciting campaign
Millions of people have been voting from coast to coast.
After what has been widely seen as an exciting campaign so far, turnout was expected to be high.
WHAT IS SUPER TUESDAY?
However voters were forced to take shelter amid an outbreak of tornadoes across Arkansas, Tennessee and Mississippi.
At least seven people were killed in Arkansas and up to 30 were injured across the three states, the Associated Press news agency reported.
"It's been a wild night," said state emergency management spokesman Tommy Jackson. "A heck of a way to have elections in Arkansas."
Among Mr McCain's supporters at the Biltmore Hotel in Phoenix, Arizona, there was a mood of enormous optimism, said the BBC's David Willis.
"I think we must get used to the idea that we are the Republican Party front-runner for the presidency of the United States and I don't really mind it one bit," Mr McCain told them.
His main challenger had been seen as Mr Romney, who won his home state of Massachusetts, as well as Utah and Minnesota.
Hillary Clinton fought back after early losses
However it was Mr Huckabee who took the first state to announce a result, West Virginia, and went on to confound the polls which had put him in a firm third.
Projections showed he won across the south, in Arkansas, where he used to be governor, Alabama, Georgia and Tennessee.
Mr Huckabee hailed his "wonderful, wonderful win here at home" in Arkansas, saying: "We are still on our feet and much to the amazement of many we are getting there folks, we are getting there."
Mr Romney said he was not about to drop out of the race. "This campaign's going on... We're going all the way to the White House," he told supporters in Massachusetts.
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