Six Celtic fans in Valencia have been arrested following altercations with police officers on Wednesday night, according to Spanish police.
A group of Hoops supporters were involved in violent clashes with local police in the city centre on Wednesday with footage showing fans walking down a street in the Spanish city, monitored by police in full riot gear.
The Jefatura Superior de Policía, police headquarters in Valencia, confirmed on Thursday that five supporters were arrested for disorderly conduct, while one more was detained for alleged drug trafficking.
Earlier on Thursday, Valencia confirmed in a statement that their Mestalla matchday ticket office would remain shut in the best interests of fan safety.
"For safety reasons, Valencia CF have taken the decision not to open the ticket office at Mestalla today," it said.
"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."
Thousands of Celtic fans are in the Spanish city for the return leg following their 2-0 defeat last week, although the 2,500 away ticket allowance is sold out.
Celtic fans have been informed that there will be no fanzone for supporters without a ticket in Valencia.
The Spanish Government's Anti-Violence Commission announced earlier this month they deemed the game a "high risk" encounter, meaning there is likely to be a hugely increased police and security presence at the tie.