Five fans are understood to have been detained for disorderly conduct, while a sixth was held for alleged drug trafficking. The arrests are thought to have come after altercations with local police officers on Wednesday night.
A large group of Hoops fans were involved in violent clashes with city cops with footage posted on social media showing supporters being followed by police kitted out in full riot gear.
An estimated 10,000 Celtic fans are thought to have travelled to the region. All 2,500 away tickets have been snapped up but there is a strong chance some travelling fans may have managed to acquire briefs for the home end.
The Jefatura Superior de Policia - the Police HQ in the eastern Spanish city - confirmed the arrests on Thursday.
The Spanish government’s Anti-Violence Commission labelled the knockout clash a “high risk” fixture, meaning there is a strong chance of a large police and security presence at the Mestalla Stadium for the clash.
Valencia also confirmed the club’s matchday ticket office at the stadium would remain closed on Thursday “in the interests of fan safety”.
A statement read: “For safety reasons, Valencia CF have taken the decision not to open the ticket office at Mestalla today.
“Valencia CF apologise for any inconvenience that this measure, which has been taken with the knowledge of UEFA, the police and Celtic FC in the interests of fan safety, may cause some of our supporters.”