WASHINGTON/NEW YORK - After a long and exceptionally negative campaign, millions of Americans voted on Tuesday for their next president as opinion polls showed Democrat Hillary Clinton with a narrow lead over Republican Donald Trump.

In a battle focused on the character of the candidates, Clinton, 69, and Trump, 70, accused each other of being unfit to lead the United States against challenges like an arduous economic recovery, Islamist militants and the rise of China.

As he did during the campaign, Trump on Tuesday raised the possibility 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.

Trump also sued the registrar of voters in Nevada’s Clark County over a polling place in Las Vegas that had been allowed to remain open late last week during an early voting period to accommodate people, many of them Hispanic, who were lined up to cast ballots.

Clinton, aiming to become the first woman US president, cast her ballot at an elementary school near her home in Chappaqua, New York early on Tuesday morning.

“It is the most humbling feeling. I know how much responsibility goes with this. So many people are counting on the outcome of this election, what it means for our country. And I’ll do the very best I can if I’m fortunate enough to win today,” Clinton said.

Trump, in his first bid for elected office, received a mixture of cheers and jeers as he arrived to vote at a school in Manhattan.

New York City was high security alert as both the candidates have arranged victory parties 20 blocks apart from each other when results start pouring in.

Clinton led Trump, by 44 percent to 39 percent, in the last Reuters/Ipsos national tracking poll before Election Day.

A Reuters/Ipsos States of the Nation poll gave Clinton a 90 percent chance of defeating Trump and said she was on track to win 303 Electoral College votes, with 270 needed for election.

Financial markets, betting exchanges and online trading platforms largely predicted a Clinton win, although Trump’s team says he can pull off an upset victory like the June “Brexit” vote to pull Britain out of the European Union.

Trump’s candidacy embodied an attack on America’s political establishment. Clinton represented safeguarding the political order.

A Clinton presidency would likely provide continuity from fellow Democrat Barack Obama’s eight years in the White House, although if Republicans retain control of at least one chamber in Congress more years of political gridlock in Washington could ensue.

A win for Trump could shake some of the basic building blocks of American foreign policy, such as the NATO alliance and free trade, and reverse some of Obama’s domestic achievements such as his 2010 healthcare law.

Polls will begin to close at 7 pm Eastern Time (0000 GMT on Wednesday), with the first meaningful results due about an hour later. US television networks called the winner of the 2008 and 2012 presidential elections at 11 pm. (0400 GMT) or shortly after.

Victory in US presidential elections is earned not by the popular vote, but by an Electoral College system that awards the White House on the basis of state-by-state wins, meaning a handful of states where the race is close assume an outsized importance.

Majorities of voters in opinion polls viewed both candidates unfavourably after a marathon campaign that began in early 2015.

“They’re both not good candidates, but I’d rather vote for ‘worse’ than ‘worser,’” said Estefani Rico, 20, a first-time voter who cast her ballot for Clinton at a library in Miami. “It’s nerve racking that in my first time being able to vote I get the worst candidates ever,” she said.

Trump and Clinton were seeking to succeed Democrat Obama, who served two four-year terms in the White House and is barred by the US Constitution from seeking another term.

Clinton spent eight years in the White House as US first lady, the wife of President Bill Clinton, from 1993 to 2001 before serving as a senator and as Obama’s secretary of state. She also ran for president in 2008, but lost to Obama for the Democratic nomination.

Trump has vowed to crack down on illegal immigration and end trade deals he says are harming US workers.

Trump seized the spotlight time and again during the campaign with provocative comments about Muslims and women, attacks against the Republican establishment and bellicose promises to build a wall along the US southern border with Mexico to stem illegal immigration.

Special Correspondent/Agencies