Borussia Dortmund chief Hans-Joachim Watzk said he had underestimated demand for seats for the first leg at the Bundesliga side's Signal Iduna Park stadium on April 24.
Brawls broke out in various parts of Dortmund city centre, with tickets sold out within an hour of going on sale.
Organised gangs tried to jump into lines of fans organised by police, with fights breaking out in the early hours of Tuesday morning.
Some fans had been queuing for tickets since Sunday evening.
"We had honestly underestimated this incredible stampede, it's our fault," said Watzke "We will learn our lessons from this and not offer a free-for-all ticket sale in future."