As millions of Americans prepared to see the two major presidential candidates debate at Hofstra University Monday, reporters were working behind the scenes to cover it, protesters were gathering outside, and law enforcement officers were on hand to keep everyone safe.

Around an hour before the debate's scheduled start time, police announced the arrests of 24 people -- 23 for allegedly sitting down and blocking Earl Ovington Boulevard and one accused of trying to jump a fence on Hempstead Turnpike onto campus. 

Hundreds of media outlets from around the world sent an estimated 3,000 journalists to cover the event, and they gathered in what's called "Media Row" to stage their coverage outside the David S. Mack Sports and Exhibition Complex, which is the site of the debate. For some reporters, the event is like a reunion with colleagues from other media outlets who haven't seen each other in years.

Outside the walls, police expected around 10,000 protesters, and thousands of law enforcement officers were on hand for security.

They brought along bomb-sniffing dogs, metal detectors, radiation detectors, the Secret Service, state troopers, and 1,000 members of the Nassau County Police Department on duty.

Some were heavily armed and visible, and others were undercover.

The planning was going on for two months. Acting Nassau Police Commissioner Thomas Krumpter says police vetted numerous tips and tracked down possible threats. He says there were no specific threats against the debate.

To help keep an eye on all that is going on, Nassau police also used a new piece of technology known as a helo-stat balloon.

"That has a two-and-a-half-pound camera on it and provides a very high-quality video that will be down fed into the operations center, along with the video that will be down fed in the helicopters," Krumpter says.

The security cost the county an estimated $1 million to $2 million. County Executive Ed Mangano says he will seek federal reimbursement to cover some of the cost, but the county did not get any in 2008 or 2012, when Hofstra hosted other presidential debates.

There were also expectations that the debate between Democrat Hillary Clinton and Republican Donald Trump could be the most viewed presidential debate in television history.

Near the staging area is another space designated for media use, "Spin Alley." Reporters watched the debate live from there to report instantly on what was going on.