The New York Police Department (NYPD) and Federal Bureau of Investigation (FBI) are investigating an alleged al-Qaeda terror threat on the eve of the presidential election.

With Americans heading to the polls on Tuesday, US intelligence officials alerted joint terrorism task forces of potential threats to New York, Texas and Virginia.

“We are aware of the information. We have been working with the FBI through the Joint Terrorism Task Force and our Counterterrorism and Intelligence Bureaus,” the NYPD said in a statement.

An officer patrols the busy Times Square on Friday. Photo: AAP

“We continue to encourage anyone with any information that could be relevant to the safety of our city to contact law enforcement.”

Federal officials declined to offer more information about the nature of the threat, but reportedly classified it as “unspecified”.

On Friday local time, officers from the NYPD anti-terror unit were seen patrolling Times Square.

New York City Mayor Bill de Blasio told WNYC radio he had learned of the situation a number of days ago, and that officials were still “assessing [the threat’s] credibility”.

“It is not at all clear how credible this is,” he said.

“We’re certainly in a vigilant position.”

The warnings, which have trickled down from federal officials to local and state authorities, come as New Yorkers gear up for the annual New York Marathon on Sunday.

‘New York is often a threat’

Mayor de Blasio told WNYC radio the city’s police presence would be ramped up on election day regardless of any terror threat.

“We have more state police, more National Guard, more soldiers on duty than ever before,” he said.

“So the bad news is New York is often a threat.

“The good news is we’ve been dealing with this now for over a decade and we have prepared for it.”

The streets of New York will be packed during Sunday’s marathon. Photo: AAP

The Port Authority of New York and New Jersey, which controls the city’s many tunnels, airports and bridges, told the New York Post it was aware of the threat and would continue to operate with “the same high level of security” in place for recent months.

The threat is made all the more serious by the fact that every American of age is being encouraged to get outside and vote on Tuesday, while as many as 50,000 runners and walkers will complete the New York Marathon – the world’s largest – on Sunday.

In 2013, three people died and over 200 others were injured when terrorists detonated bombs at the finish line of the Boston Marathon.

Threat follows cyber concerns

The ongoing investigation comes two days after American news station NBC claimed the US government was becoming increasingly concerned Russian hackers were planning on interfering with the upcoming election.

According to the report, the White House and Department of Homeland Security were coordinating in an unprecedented effort against the hackers.

Donald Trump has denied Russian hackers want to rig the election. Photo: Getty

Officials are reportedly preparing for anything from social media propaganda to a complete shutdown of the power grid, or a last-minute dump of documents that implicate one of the candidates.

Democratic candidate Hillary Clinton has maintained throughout the late stages of her campaign that Russia is gunning for Donald Trump to enter the White House, and that Russian hackers are behind the released of classified documents and emails that have reflected poorly on Ms Clinton.