Polling stations across nine states on the east coast opened at 6:00 am local time (1100 UTC) on Tuesday. 

Voters in Connecticut, Indiana, Kentucky, Maine, New Hampshire, New Jersey, New York, Vermont and Virginia got first crack at electing the new president.

Polling stations continued to open across the US - all the way westward to Hawaii - throughout the day. 

According to Associated Press data, about 45 million Americans cast their ballots ahead of Election Day by early voting either by mail or at polling stations.

The latest polls have Democratic candidate Hillary Clinton slightly ahead of Republican Donald Trump. However, the polls anticipate a tight race in vote-rich states such as Michigan and Pennsylvania.

The first exit polls are expected to around 0000 UTC. However, it may take several hours before the direction of the race becomes clear.

Even then, it remains unclear whether Trump will concede defeat should he lose, having repeatedly warned that a "corrupt Washington and media elite" is seeking to rig the race.

Donald Trump's campaign filed a lawsuit in a Nevada court alleging polling place "anomalies" during early voting in the Las Vegas area last week. The claim centered on an accusation that a polling place had improperly been allowed to remain open.

In a telephone interview on Fox News Tuesday, Trump once again raised the possibility of not accepting the election outcome.

"We're going to see how things play out today. Hopefully they'll play out well and hopefully we won't have to worry about it, meaning hopefully we'll win," Trump said. "I want to see everything honest."

Candidates cast their ballots

Clinton cast her ballot early Tuesday morning at a school near her home in Chappaqua, New York.

"It is the most humbling feeling. I know how much responsibility goes with this," Clinton said. "So many people are counting on the outcome of this election, what it means for our country." 

Trump began Election Day with a call to the morning news show "Fox and Friends." He cast his vote in Manhattan.

"It's been a beautiful process. The people of this country are incredible," Trump said of the election. However, he added, "If I don't win, I will consider it a tremendous waste of time, energy and money."

The Democratic vice presidential nominee, Tim Kaine, cast his ballot in this hometown of Richmond, Virginia, at a retirement community center near his home.

Speaking to reporters afterwards, Kaine urged Americans to get out and vote, and promised that he and Clinton would try to bring the country together should they be elected to the White House.

"The sign of a vigorous democracy is one where a lot of people participate," Kaine said.

Trump's running mate, Indiana governor Mike Pence, cast his vote in Indianapolis. He and his wife, Karen Pence, went on a bike ride before walking to a polling station. 

No selfies in the polling booth

On Tuesday Donald Trump's second son, Eric Trump, may have broken New York state law by tweeting a photo of his filled-out ballot. The image, which was later deleted from Eric Trump's Twitter account, showed a ballot with the oval under Donald Trump's name filled in. It was accompanied by the words "It is an incredible honor to vote for my father! He will do such a great job for the U.S.A!" Posting such an image, also known as a "ballot selfie" is in violation of New York state law. It was unclear whether he would face legal consequences from posting the image. 

A number of states have told voters to put down their phones and leave their selfie sticks at home when going to the polls. Twenty-four states have laws against disseminating photos and videos of the ballot.

Such laws date back more than 100 years when their intention was to prevent coercion, intimidation and vote buying. However, the laws have raised concerns around the First Amendment in today's social media and selfie-era.

Last week in California, a lawsuit was filed arguing that the ban violates voters' freedom of speech and political expression. A federal judge refused to lift the ban, citing that a change in the law so close to the election would create confusion. The law will be lifted in January instead.

A similar discussion is taking place in Colorado. Under the current law, anyone found to be sharing photographs of their ballot risks being fined or even faces prison. However, state election officials reportedly don't know of any instances when people have been charged for disseminating a completed ballot.

The law has come under public scrutiny after singer Justin Timberlake posted a photo on Instagram of himself in the polling booth. In Timberlake's home state of Tennessee, taking photographs or video in the polling booth is prohibited and can incur a 30-day jail sentence.

Instead of decrying the singer, however, the Tennessee secretary of state's office used it as opportunity to remind the state's voters to forgo their electronic devices while fulfilling their civic duty.

German politicians closely watching vote 

German Chancellor Angela Merkel, who has refrained from commenting directly on Clinton or Trump, said Tuesday she was awaiting the result of the US vote "with suspense." Asked about the possibility of a woman winning the White House, Merkel said: "Then we'd come a little bit closer to a balance of women and men in leading positions."

Merkel was speaking alongside Norwegian Prime Minister Erna Solberg, who said many young women could be inspired to see politics not just as something that belongs to men, before adding "There isn't some kind of global girlfriends' network that wants to rule the world."

German Foreign Minister Frank-Walter Steinmeier said both Germans and many Americans were "glad that this special election campaign is coming to an end." Speaking in Berlin on Tuesday, Steinmeier said it would be difficult for the incoming president to bridge the divisions caused by the campaign.

dm, se/kms, kl (Reuters, AP)