Protesters began to arrive at designated areas around Hofstra University on Monday afternoon, hours ahead of the evening’s presidential debate. But Nassau County’s police commissioner said law-enforcement officials are aware of “no known viable threats.”

On Monday afternoon, police escorted Green Party candidate Jill Stein off campus and gave her a ride to her nearby hotel, officials said. Stein had gained credentials to come on campus to do a TV interview, but once that was over, she had to leave, said Det. Lt. Richard LeBrun, a department spokesman.

Acting Police Commissioner Thomas Krumpter has estimated as many as 10,000 protesters could gather in designated “free speech” zones outside of the Hempstead campus ahead of the debate between Republican Donald Trump and Democrat Hillary Clinton.

More than 1,000 sworn officers, including “a significant number of undercover officers in plainclothes and suits,” were on hand in preparation for Monday night’s debate, Krumpter said.

Snipers from the department’s Bureau of Special Operations were on rooftops, and uniformed officers lined Hempstead Turnpike.

“People have the right to exercise free speech and we’ll protect that right — but by the same token, violence won’t be tolerated, trying to breach security won’t be tolerated,” Krumpter said Monday afternoon as he stood on Hempstead Turnpike outside the university’s main entrance.

Krumpter said officers from State Police, NYPD, the Metropolitan Transportation Authority, Suffolk, and Hempstead and Garden City villages also are on hand, as well as the FBI, ATF and Secret Service.

Among the security technology is a helium balloon holding a video camera stationed hundreds of feet in the air above the free-speech zone.

Krumpter said video is fed into the department’s on-site operations center, giving police a bird’s-eye view into the demonstration area.

The “free-speech zone” for protesters is located on the South Campus, across Hempstead Turnpike, and protesters will be screened for prohibited items including weapons, mace, pepper spray and selfie sticks, police said.

“This is not what democracy looks like, but they are being very nice,” said Stein, who added she would be back later for another interview.

Krumpter earlier had said Stein would be treated like any other demonstrator and would be arrested if she breached security.

“Dr. Jill Stein believes that she gets media stories by getting arrested,” Krumpter said. “Hopefully, today she’ll think twice about it. But if she tries to force her way in, we’ll be more than happy to accommodate her and arrest her.”

Stein and Libertarian candidate Gary Johnson were not permitted to participate in the debate by the Commission on Presidential Debates, because neither garnered at least 15 percent support in polling leading up to the debate.

Kenneth Mulvena, a Hofstra junior, early Monday afternoon said he planned to gather with supporters of Libertarian candidate Gary Johnson to protest Johnson’s exclusion.

“I think it’s a travesty,” said Mulvena, 20, of Bellerose, Queens. “The lesser of two evils is not something the American people should stand for.”

Meanwhile, road closures around campus have begun ahead of the debate.

Charles Lindbergh Boulevard and Earle Ovington Boulevard will be closed until midnight, as will Hempstead Turnpike from Oak Street to Merrick Avenue near Eisenhower Park. Oak Street also will be closed from Hempstead Turnpike, northbound, to Westbury Boulevard.

California Avenue and Meadowbrook Place will be closed, northbound, at Belmont Place. Uniondale Avenue and Manor Parkway will be closed, headed north, at McKenna Place.

Nassau County officials are urging drivers to use other eastbound or westbound routes: Front Street, Old Country Road or Stewart Avenue.

Lawrence Street in Uniondale will become a one-way street, heading north from Hempstead Turnpike to Westbury Boulevard; Courtenay Road in Hempstead can be accessed one-way, headed south from Hempstead Turnpike to Front Street.

Manor Parkway, Marvin Avenue, Walton Avenue, Gilroy Avenue and Cunningham Avenue will become one-way streets, for one block south of Hempstead Turnpike.

There will be no parking on Oak and Lawrence streets, Courtenay Road and McKenna Place.

Traffic delays and updates will be projected on six electronic portable message signs.