[NEW YORK] Republican Donald Trump and Democrat Hillary Clinton waged a tight battle in several crucial battleground states on Tuesday in their bitter race for the White House, although opinion polls showed Mrs Clinton had an edge in the closing hours of the campaign.

With voting completed in more than half of the 50 US states, the race was too close to call in Ohio, Florida, North Carolina, New Hampshire and Virginia, states that could be vital to deciding which contender wins the presidency.

Both candidates scored early victories in states where they were expected to win. Mr Trump captured conservative states in the South and Midwest, while Mrs Clinton swept several states on the East Coast and Illinois in the Midwest.

Those victories were long predicted and not especially significant in the national race, which is likely to turn on a half-dozen toss-up states that will be crucial in the state-by-state fight for 270 Electoral College votes needed to win.

Mrs Clinton had more options to reach 270, with Mr Trump needing a virtual sweep of about six toss-up states to win.

Mrs Clinton led Mr Trump, 44 per cent to 39 per cent, in the last Reuters/Ipsos national tracking poll before Election Day. A Reuters/Ipsos States of the Nation poll gave her a 90 per cent chance of defeating Mr Trump and becoming the first woman elected US president.

In a campaign that focused more on the character of the candidates than on policy, Mrs Clinton, 69, a former US secretary of state, and Mr Trump, 70, a New York businessman, accused each other of being fundamentally unfit to lead the country.

Mr Trump again raised the possibility on Tuesday of not accepting the election's outcome, saying he had seen reports of voting irregularities. He gave few details and Reuters could not immediately verify the existence of such problems.

In North Carolina, the state elections board extended voting hours in eight Durham County locations after technical errors led to long waits.

Financial markets, betting exchanges and online trading platforms largely predicted a Clinton win, although Mr Trump's team said he could pull off a surprise victory like the June "Brexit" vote to pull Britain out of the European Union.

The economy, terrorism and healthcare ranked as the top three concerns facing Americans casting ballots in Tuesday's election, according to an early reading from the Reuters/Ipsos Election Day poll.

The poll of about 35,000 people found that 25 per cent of voters picked the economy as the "most important problem". Another 14 per cent named "terrorism/terrorist attacks" and 13 per cent picked healthcare.

By contrast, the economy was the No 1 concern for 46 per cent of American voters in 2012, according to the Reuters/Ipsos national tracking poll four years ago. Almost nobody listed terrorism as a top concern in 2012, and 8 per cent listed healthcare as the top worry.

A signature Trump issue, immigration, was chosen by 7 per cent of voters as the most important issue in Tuesday's poll.

Some 15 per cent of Americans who cast a ballot on Tuesday said it was their first time voting in a presidential election, up from 9 per cent in 2012, according to the Reuters/Ipsos Election Day poll. The poll showed 13 per cent of voters waited until the final week to make up their minds.

Also at stake on Tuesday was control of Congress, with Republicans defending a slight four-seat majority in the 100-member Senate. The House of Representatives, where all 435 seats were up for grabs, was expected to remain in Republican hands.

The Dow Jones Industrial Average index ended up 0.4 per cent as investors bet on a win for Mrs Clinton, seen by Wall Street as more likely to ensure financial and political stability.

Mexico's peso hit a two-month high on Tuesday on the expectation of a loss for Mr Trump, who has vowed to rip up a trade deal with Mexico.

In the closing stages of the race, the two campaigns focused on several crucial battleground states, including Ohio, Pennsylvania, North Carolina and Florida, as they tried to piece together the 270 electoral votes needed to capture the White House.

Majorities of voters told pollsters they viewed both candidates unfavourably after a particularly bruising and divisive campaign that began in early 2015. For months, polls showed both Mrs Clinton and Mr Trump were unpopular, although Mr Trump was more so.

REUTERS