US President Obama thanks supporters on polling day

Tens of millions of Americans are voting to decide whether to re-elect Democratic President Barack Obama or hand the job to Republican Mitt Romney.

Polling stations will start closing in eastern states at 18:00 EST (23:00 GMT) and a winner could become clear within hours.

Turnout is crucial, with polls suggesting a neck-and-neck race, but giving the president a slender lead in crucial swing states.

Mark Mardell reports from Chicago, where Mr Obama will be waiting for the results.

For in depth coverage as the US goes to the polls: BBC US election special report