Shocking moment random man smacks bar patron over the back of the head with a huge tree branch

Surveillance footage outside a downtown Dallas bar caught the shocking moment a random man smacked a patron over the back of his head with a large stick before fleeing.

In the footage, an unidentified man could be seen stopping in front of an outdoor section of City Tavern just before 2am on Saturday and raising a large stick like a baseball bat.

He then slammed it into the back of a man's head, causing the victim to topple over.

The victim, later identified as Adam Mercado, could then be seen grabbing his head and rolling over in a fetal position.

Mercado's girlfriend, Taylor Monnet, immediately rushes over to check on him - then runs back into the bar to call for help, as the assailant punched a City Tavern employee in the face and makes his escape. 

Dallas police are now searching for the suspect, but say they only have the security footage to work with, according to WFAA.

To help, Mercado is asking everyone in Dallas review the security footage to see if they recognize the assailant -  and be on the lookout for him in the future.

'I always have my guard up,' he said. 

'Make sure you keep your head on a swivel - because for 10 seconds, I let my guard down and this happened.'

He has said he and Monnet went out to City Tavern to meet up with friends for last call on Saturday morning, when he first spotted the suspect.

'I noticed a man standing off to the side of the entrance, watching the patrons on the patio, holding a tree branch that had been whittled into a spear at one end,' he wrote in an online fundraiser for his medical bills.

He said he immediately knew something was wrong.

'Initially, my senses were going crazy, my intuition was going crazy, so I said, "You know what, let's go inside and get a drink,"' he told NBC DFW.

The couple eventually decided to head home just before 2am - and when they stepped out onto the patio, 'That's when I felt it,'  Mercado said.

'I let my guard down for about 10 seconds and that's all it took - just a WHACK.' 

'The sound was loud enough to almost be a gunshot,' he noted. 'That's what scared me.' 

As he fell to the ground, Monnet said she 'didn't know what to do.'

'So I'm running into the bar and I'm screaming, just begging for help from anyone.'

The couple said police and EMTs arrived to the scene, but they turned down an ambulance ride because they were concerned about the cost.

A few hours later though, Mercado said he took himself to the hospital 'because my head, my ears, everything was ringing, I was sore. And I knew that I had to get checked out.'

Mercado wound up spending six hours in the emergency room, and was eventually diagnosed with a concussion.

Over the next few days, Mercado said he was left with lingering pain and brain fog. 

He said he was also afraid to leave his apartment out of fears he could be randomly attacked again.

'It's just been really traumatic, you know, I've never had any type of violence done unto me,' he said, adding that the experience was 'very surreal.'

He said he is 'grateful and thankful that I'm alive, and I hope this never happens again.' 

Mercado is also asking for help to pay for his medical expenses, writing in his online fundraiser that he is 'now faced with medical bills that I cannot afford, accompanied by wages lost over the last few days.'

That fundraiser has netted over $6,000 by Thursday morning.

DailyMail.com has reached out to Dallas police and City Tavern for more information.