Thousands flocked to polling stations here and across the United States as the high-stakes Super Tuesday contest began after a final day of rallies marked by ugly encounters.

"There were 20 people in line at 6am. I've been voting here for 40 years and I don't remember it ever being this busy," said Democratic Party volunteer Donna McLean, who was out in front of a school handing out fliers.

Similarly, Mr Jack Powers of the Alexandria Electoral Board said the turnout has been higher than in 2008 when then senator Barack Obama was on the ballot. "I think heavily contested nominations always bring out more voters and Virginia is considered a swing state in this election," he said.

Though Virginia does not have the biggest number of delegates among the 13 states voting in primary elections on Super Tuesday, it is likely the only swing state in the November general poll for president.

The state, which borders the capital Washington, DC, is also seen as a key one for the Republican establishment favourite, Senator Marco Rubio, in his bid to halt the rise of billionaire Donald Trump. The senator has significant support among Republicans in the state's north and he is hoping it can edge out Mr Trump's advantage in the more rural southern part of the state.

The heightened interest was clear a day earlier as thousands flocked to election rallies, even though they were held in the middle of a working day.

An estimated 11,000 were left outside the Trump rally in Radford and a line stretching nearly a kilometre queued to get into the rally by former secretary of state Hillary Clinton in Fairfax on Monday.

Nerves were sometimes frayed - a woman wearing a Hillary T-shirt was in tears as she screamed at a campaign staff member - as thousands were turned away from auditoriums packed to capacity for the rallies.

In all, it was a frantic, often ill-tempered last day of campaigning for the single biggest day of voting in the election primary season.

Much of the drama on Monday came from Republican front runner Donald Trump. Though various candidates have faced hecklers and protesters at rallies, none faced the same amount of disruption as Mr Trump did in Radford, Virginia.

Mr Trump had to pause during his speech on three different occasions to ask security personnel to escort protesters out of the packed university auditorium.

"Get them out, please. Get them out, get them out," Mr Trump said from the podium during one interruption. "Right smack in the middle of my punchline."

In a sign of how tensions have spiked as the stakes in the race increase, some Trump supporters yelled abuse at the protesters being escorted out.

A 15-year-old who attended the rally told The Straits Times that she did not feel safe. "It was crazy inside... People were shouting profanities at others and I was nervous the entire time," said high school student Emilia Scarpa-Friedman.

Four hours away in Fairfax, Mrs Clinton's rally at George Mason University saw its fair share of conflict.

A handful of protesters turned up as people queued to get in. College student Jason Dukakis, 21, held up a sign that read: "Unsecured e-mails sink corrupt females."

Both Mr Trump and Mrs Clinton currently lead the polls for the states that vote on Super Tuesday.

A good performance by Mrs Clinton would strengthen her position as the presumptive nominee of the Democratic Party. She is riding high after a resounding victory over Senator Bernie Sanders in South Carolina. For Mr Trump, the results will be watched for whether the urgent effort by the Republican Party establishment to stop him is bearing fruit.