A Secret Service spokesperson said in a statement there was a commotion in the crowd and an "unidentified individual" shouted "gun," though no weapon was found after a "thorough search."
A man, who later identified himself to reporters as Austyn Crites, was then immediately detained and led out by a throng of police officers, Secret Service agents and SWAT officers armed with assault rifles to a side room.
A law enforcement official later told CNN no charges were filed against Crites.
After he was released from custody, Crites told reporters the incident started off when he raised a "Republicans Against Trump" sign.
Crites said he was then assaulted by a group of people around him before anyone shouted anything about a gun.
"All of a sudden, because they couldn't grab the sign, or whatever happened, bam, I get tackled by all these people who were just, like, kicking me and grabbing me in the crotch and just, just beating the crap out of me," Crites said, according to KTNV. "And somebody yells something about a gun, and so that's when things really got out of hand."
Crites told ABC News Sunday he "just wanted to voice my displeasure," and said he has no association with the Clinton campaign, other than personally supporting the Democratic nominee.
"I was a Republican supporter through the primaries, and I have donated money to the Hillary Clinton campaign recently because I think that Trump is a disaster for the country," he said, adding he has already voted for Clinton.
The alleged assault against Crites is just the latest such incident to occur at a Trump rally, where other protesters have previously been roughed up.
Trump was unharmed and returned to the stage minutes later to finish his speech.
"Nobody said it was going to be easy for us, but we will never be stopped. We will never be stopped. I want to thank the Secret Service. These guys are fantastic," Trump said, before returning to his stump speech.
Trump was in the middle of his stump speech when the commotion occurred. He was looking into the crowd, his hand over his eyes to block the glare from the stage lights, when Secret Service agents grabbed him and escorted him off the stage. Trump ducked his head as he left the stage.
The crowd surged backward, some supporters with frightened looks on their faces, as the Secret Service and police tactical units rushed in to detain a man.
Several attendees told CNN they initially heard a person yell there was someone with a gun.
"We were all very interested in what Mr. Trump had to say and a guy in a red shirt just rushed right next to me, screaming, 'There is a guy with a gun,'" Shimon Cohen told CNN.
But asked if he saw a gun, he said, "No, I did not."
A second person, Milton Zerman, said he "was watching Trump speak and I heard someone scream, 'This guy has a gun,' so I looked toward the guy he was talking about. It didn't look to me that he had a gun -- or at least I didn't see a gun -- but people were screaming that he did have a gun and immediately after that Trump was taken off stage.
"Everyone starts dispersing, I hear people saying, 'Let's get out of here, it's not going to be good,' but I just took that opportunity to get closer to the front because it seemed like the police officers had it under control."
Trump held a last rally Saturday night in Denver, where he did not address the incident.
Before his last rally, Trump issued a statement about the commotion in Reno.
"I would like to thank the United States Secret Service and the law enforcement resources in Reno and the state of Nevada for their fast and professional response," he said. "I also want to thank the many thousands of people present for their unwavering and unbelievable support. Nothing will stop us -- we will make America great again!"
Less than an hour after the incident -- and before it was clear what happened -- Trump's campaign sought to highlight the moment to suggest he was tougher than his Democratic rival, Hillary Clinton, who had to cut short a Florida rally due to heavy rain.
Dan Scavino Jr., Trump's social media director, and Donald Trump Jr., the nominee's oldest son, retweeted Jack Posobiec, a supporter who wrote: "Hillary (Clinton) ran away from rain today. Trump is back on stage minutes after assassination attempt."
But the Secret Service statement made clear no weapon had been found. Neither has deleted the retweet.
Secret Service agents rushed to shield Trump onstage in March after a man tried to rush the stage during a rally in Dayton, Ohio, but the GOP nominee wasn't escorted off stage.
And a man was arrested at a rally in Las Vegas in June after he tried to grab a police officer's gun, which he said he planned to use to shoot Trump