Democrat Hillary Clinton and Republican Donald Trump have both taken early leads in their respective contests for the US presidential nomination.
Alabama, Georgia, Tennessee and Virginia were among the states both candidates won. Mr Trump was defeated by Ted Cruz in Texas and Oklahoma.
Democratic candidate Bernie Sanders has won his home state of Vermont.
Super Tuesday sees 11 states voting on the biggest single day ahead of the 8 November presidential election.
Former Secretary of State Hillary Clinton and real estate mogul Donald Trump entered Super Tuesday as the favourites to win the vast majority of states for their respective parties.
Mrs Clinton has also taken Arkansas and Texas, and Mr Trump has won Massachusetts.
Early exit polls show Mr Trump and Ohio Governor John Kasich leading the Republican field in Vermont.
Mrs Clinton and Mr Sanders are neck and neck for the Democrats in the north-eastern state of Massachusetts.
Results from the remaining states are expected in the coming hours.
Mr Trump has stunned the Republican establishment to become the party’s front-runner. Despite his controversial policies on immigration, the former reality TV star has been consistently polling well above his rivals – Ted Cruz, Mr Rubio, Mr Kasich and retired neurosurgeon Ben Carson.
Both Senators Cruz and Rubio have ramped up their anti-Trump rhetoric over the past week, in a bid to halt his commanding lead in the race. The outcome of Super Tuesday will be critical for both candidates to remain the race.
On the Democratic side, Mrs Clinton had already secured three wins in the first four early-voting states and has led significantly among blocs of black voters there.
Bernie Sanders has put up an unexpectedly strong challenge against the former secretary of state after his sweeping victory in New Hampshire last month.
The Vermont senator and self-described democratic socialist is hoping to win in several other states on Tuesday such as Massachusetts, Oklahoma and Minnesota.
Addressing cheering supporters at his victory speech in Vermont on Tuesday, Mr Sanders aimed a jibe at the Republican front-runner saying: “We are not going to let the Donald Trumps of the world divide us.”
Voters have been casting their ballots across the country, from Massachusetts and Virginia on the east coast to Texas and all the way to Alaska.
Super Tuesday is pivotal in the presidential nomination race, with nearly a quarter of the 2,472 Republican delegates are up for grabs and some 20% of all delegates for the Democrats.
Follow the primaries race with the delegate tracker, provided by the Associated Press (AP)
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