US Presidential Election: Voters cast their ballots early

As the US presidential election race entered its final hours on Monday, people across the country turned up to vote early.

Queues of people waited to cast their vote at early-voting centres in Ohio, Florida and Indiana.

Some 30 (m) million people have already voted in 34 states and the District of Columbia, either by mail or in person, although none will be counted until Election Day on Tuesday.

Tuesday's election appears to be going down to the wire, though most opinion polls indicate that President Barack Obama may have the edge over Mitt Romney in the crucial swing states.

Obama and Romney are undertaking a final exhaustive campaign push that won't end until the early hours of Tuesday in pursuit of every possible vote.

Ohio is the only state both candidates planned to be in on election eve, with their rallies being held just seven miles apart.

Under the US system, the winner is not determined by the nationwide popular vote but in state-by-state contests - making nine "battleground" states that don't consistently vote Republican nor Democratic extremely important in such a tight race.

Winning a state gives Romney or Obama that state's electoral votes, which are apportioned to states based on a mix of population and representation in Congress.

No Republican has won the White House without also winning in Ohio and Republicans - and Democrats alike say Obama seems in slightly better shape than Romney in Ohio and Wisconsin.

A final national NBC/Wall Street Journal Poll showed Obama getting the support of 48 percent of likely voters, with Romney receiving 47 percent.