Arizona, California, Connecticut, Delaware, Florida, Illinois, Maryland, Missouri, New Hampshire, New Jersey, New York, Ohio, Oklahoma, Pennsylvania, Rhode Island, South Carolina, Texas, Vermont, Virginia, Washington state, Wisconsin
Mitt Romney 11 states, 251 delegates
Alaska, Colorado, Maine, Massachusetts, Michigan, Minnesota, Montana, Nevada, North Dakota, Wyoming, Utah
1,191 delegates needed for nomination. Source: AP (includes all kinds of delegates)
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.
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."
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?
24 states holding simultaneous contests to help decide the Democratic and Republican presidential nominations
About 40% of each party's delegates - who will choose the candidate - are up for grabs
Key states electing large numbers of delegates include California, New York and Illinois