Eyewitnesses said about 50 fans were in the Bar BQ cafe in the Belgian capital on Wednesday night when up to 100 people tried to break in.
"We had Tottenham fans and suddenly people came from outside," bar worker Marie Elizabeth told BBC Radio London.
"They started hitting people with chairs, tables and glasses - whatever they found."
One person was reported to be badly injured in the attack.
Ms Elizabeth said the atmosphere inside the bar had been "very good" before the attack, and that Spurs supporters "did not do anything to cause problems".
"It was very scary because it is the first time something like this has happened in the area," she added.
"It wasn't expected and we didn't know what to do.
"Thank god someone was by the door and they tried to close it, so they didn't come inside. If they had come inside I don't know what was going to happen.
"A lot of people from England were hurt. We called the police and the ambulance and they came for first aid."
Tottenham fan Sion Roberts tweeted pictures from outside the bar following the attack, showing displaced tables and chairs and damage to the bar's frontage.
One person was reported to be badly injured in the attack.
Fans of the north London club have previously been targeted when playing on the continent, notably in Rome in 2012 and Lyon in 2013.
Tottenham play Anderlecht on Thursday night.
A group of Tottenham fans have been attacked in Brussels before the Europa League tie against Anderlecht.