Footage has been circulating online showing police officers with shields and helmets confronting Wolves supporters last night.
Around 2,000 Wolves supporters have tickets for the match but thousands more are thought to have travelled to Barcelona where they have been enjoying the hospitality and singing Wolves chants throughout the city centre.
Some fans have criticised the policing as heavy handed and complained of being "beaten" and "dragged through the streets," while others said a small minority of Wolves fans had been fighting before officers arrived.
WATCH: Wolves fans describe riot police clashes
One video showed chairs being thrown towards police near where a man lay on the ground outside busy bars while another showed a man being hit with a baton as several officers restrained him.
Dave Randle, from Willenhall, is out in Barcelona with his godson and two friends, and said he'd "never seen anything like it".
The 50-year-old said: "It was totally unprovoked. Just barbaric.
"We were just having a few drinks in a bar off one of the main squares. At about 10.30pm it began to close so they gave us plastic glasses and we went outside.
"But then 10 police vans show up and riot police get out with shields.
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"They pushed everyone into the corner of the square. People were getting beaten, dragged about and hit with batons.
"These were just innocent people. We were singing but that was it. It had been a good atmosphere. It does make me worry about going to the game tonight.
"Wolves fans started reacting though when police began beating people with riot sticks."
It is not yet known if any arrests were made.
Well just witnessed. #spanishpolice laying into #wolves fans in the cathedral quarter. Totally wrong @expressandstar # have to see it to believe it. @Wolves @WWFCFancast Thugs in uniform. No wonder they get the reactions.totally disgusted with #Spanish police .— BurkeBrosMovingGroup (@BurkeBrosMoving) February 26, 2020
Wolves fan Vaughan Little described to the Express & Star the moment police arrived where Wolves fans had congregated
"In one of the main squares there was a load of Wolves fans, no Espanyol fans anywhere," he said. "Great time, drinking beer, singing songs, been there all afternoon.
"Early evening - half past ten, 11 o'clock - the police turned up. Maybe the bar owners wanted to close the bars, I don't know. Lads were having a great time. As soon as I saw the police I would say 60 per cent of the lads all moved away, out of the way.
WATCH: Footage shows riot police in Barcelona
WARNING: Video contains strong language
Footage courtesy of Dave Randle
"Hardcore of lads just enjoying themselves and it all got very very heavy handed I'm afraid.
"I moved away, kept out of the way, as the majority of Wolves fans did.
"A few lads probably didn't even know it was going on, had a few beers all afternoon, got pushed into a corner, what you gonna do? There's a bit of a reaction and the next thing is it's a bit heavy handed.
"I saw batons, saw screens, I didn't see anyone actually being hit. I saw three, four, five lads being taken away into the police vehicles and, as I say, the majority of Wolves fans were shaking their heads in disbelief.
"Why? There wasn't one iota of trouble."
Fellow travelling fan Vince Holmes added: "When I arrived I witnessed police with vans and cars pushing the Wolves fans back 'cause they were a bit rowdy but Wolves were having a good time, a good laugh, enjoying themselves.
"It was pathetic really, Wolves weren't doing no harm to anyone, they were enjoying a good drink and a bit rowdy but the police went in with batons hitting out, terrible."
Asked if he saw anybody being struck by police, Mr Holmes said: "Yes, I witnessed a fair few showing the bruises afterwards. One lad went down, hit on the back of the neck, he's alright now apparently but he felt that one.
"Hopefully it should go smoothly now, we'll just have to see.
The clashes do not to appear to have involved supporters of any other clubs - unlike when Wolves fans were targeted in Porto during a Europa League trip in November.
On that occasion running battles broke out when organised groups of hooligans believed to be supporters of Belgian club Standard Liege ambushed Wolves fans.
The Liege fans were en route to watching their side play Vitoria SC in Guimaraes and Wolves fans were travelling to watch the club play in Braga when they were targeted.
Many Wolves fans then missed much of the group-stage match in Braga as they faced chaotic and reportedly aggressive policing outside the stadium.
Wolves beat Espanyol 4-0 in the first leg of the round of 32 match at Molineux last week and the second leg kicks off at 5.55pm GMT on Thursday.
One Spanish supporter was arrested on suspicion of racist abuse in Molineux, while three Espanyol fans were attacked by a group of youths after the match in Wolverhampton.