Minutes after a man armed with an assault rifle opened fire at Pulse, a gay nightclub in Orlando, Fla., the venue posted a dire warning on Facebook: "Everyone get out of pulse and keep running."

It was a stark contrast to the club's previous post, seven hours earlier, inviting "Latinos, Latinas & everyone that loves a little Latin flavor" to swing by Saturday night for an evening of fun.

They didn't know then the club would become the scene of the deadliest shooting in U.S. history, with 50 people killed, 53 others injured and the gunman dead in a shootout

As the chaotic scene unfolded, staff, patrons and passersby took to social media to describe what they saw.

A man named Brandon Wolf tweeted in the midst of the shooting that he was hiding in the bathroom and unable to find his friends.

Omg. Shooting at pulse. We hid in the bathroom. And we can't find our friends. —@bjoewolf

Three hours later, he posted that he'd managed to escape the nightclub safely. Others, he said, were not so lucky.

My heart is broken. We couldn't save Juan. Thank you everyone who was praying and hoping. May he rest easy ❤️ —@bjoewolf

Another man, who describes himself in his Twitter bio as an ER physician assistant, posted a clip from outside the club.

Walking to ORMC now. I'm ok. Friend got shot. I've never seen so much blood and mayhem or been so scared ever. <a href="https://t.co/KyGhQGmWWQ">pic.twitter.com/KyGhQGmWWQ</a> —@lakecitymedman

He said he'd been celebrating at Pulse with a friend for her birthday. The pair managed to escape out the back, he said, though his friend was shot in the shoulder.

can't explain what happened. I was there with a nurse for her bday. Guy in vest startsshooting. She got hit in shoulder. We ran out back —@lakecitymedman

I'm at ORMC now after triaging people in the street. Trying to get temporary credentials to help out. Patients everywhere bleeding. —@lakecitymedman

Another man posted video from what appears to be just down the street from the club. The sound of gunfire, a fast burst of at least 20 shots, can be heard ringing out.

Gunfire exchanged at <a href="https://twitter.com/hashtag/pulse?src=hash">#pulse</a> nightclub hostage situation in <a href="https://twitter.com/hashtag/Orlando?src=hash">#Orlando</a>. <a href="https://t.co/qeF0s6Ien6">pic.twitter.com/qeF0s6Ien6</a> —@nichornstein

As the chaos died down on Sunday morning and details of the shooting emerged, tributes for the victims began pouring in on social media under the hashtags #PrayForOrlando and #ImWithYou.

'Out of nowhere, bang, bang, bang, bang'

Orlando resident Justin Barnes was stopped in his car at a red light near the nightclub when the first round of gunfire erupted.

"People were screaming and yelling and the officer that was there, he was firing his weapon at the guy and yelling 'get the f--k out of the way.' People were running past my car," he told CBC News.

"One lady was dragging her leg and she was bleeding," Barnes said. "I could literally smell gunpowder from where my car was sitting."

Justin Barnes recounts mass shooting at a gay nightclub in Orlando 9:07

Barnes said he got out of his car to try to help victims get away safely, but later fled when the gunman began shooting in his direction.

"I saw like five people drop like balloons in front of me," he said, visibly shaken and struggling to find his words during his interview with CBC's John Northcott.

"I wasn't affected by it physically, but I'm telling you man, I'll be affected by this mentally for the rest of my life. I've never seen anybody die in my life."

'I crawled out'

"I just saw bodies going down," said customer Chris Hansen, who was ordering a drink from the bar when the shooting began.

"I fell down. I crawled out. People were trying to escape out the back."

Outside, Hansen helped a man who had been shot in the back by placing a bandana on his wound to stem the bleeding. He also helped a woman who was shot in the arm.

'No one can tell me where my son is'

One woman whose son was in the club told reporters she's since texted and called his phone, with no response.

"I don't know where my son is. No one can tell me where my son is, if he's been shot, if he's dead, no one knows," she said through tears.

She said other witnesses saw her son's boyfriend being taken away from the scene in an ambulance with multiple gunshots.