Tom Leese, 34, a video producer, and his wife Rachael, 31, an account director – who are on their honeymoon in the country, were having a drink in the Slivovitz Museum, close to where the shooting occurred.
Mr Leese told the PA news agency: “A policeman came in and started shouting loudly in what I assume was Czech.
“The staff were very calm, turned all the lights off very quickly and urged us to stay calm, the restaurant was relatively quiet.
“The policeman left urgently and we stood in the corner of the restaurant.”
He said they stood there “really quite terrified” with no knowledge of what was going on, where the incident was happening and whether there was more than one shooter.
The couple, from Merstham in Surrey, remained in the museum for about an hour, choosing to stay for a while after a member of staff declared it safe.
“The strange thing is it just didn’t seem real,” Mr Leese told the PA news agency.
“Working in the news, you have these situations happen once or twice a year.
“We know that…attacks happen but you never really think that you’re going to be in the middle of one or be close to one – and then when it does happen, you kind of freeze.”
“The people in the restaurant were really calm and it really helped us sort of maintain a little bit of composure,” he continued.
“It was really really scary, but, you know, they were very good in there in helping us stay as calm as possible.”
He recalled walking up to a “shaken” member of staff afterwards, to pay for their drinks, who told them the tram stop outside, where police were, is how workers get home late at night.
The couple, who got married on Saturday, got in touch with their families and friends “straight away” and “can’t wait” to return to the UK on Friday.
They were meant to be going to a restaurant on Thursday evening “right opposite” where the attack occurred, but, as they went to call off their booking, realised the eatery had already cancelled all reservations.
Police gave no details about the victims or the circumstances of the gunfire.