Fans & players pay tribute after Borussia Dortmund Westfalenstadion tragedy
A sombre atmosphere descended on the stadium as news of the tragedy spread around both sets of fans.
Another supporter also had to be resuscitated during the game in a separate incident.
Goals from Marco Reus and Shinji Kagawa sealed the win for Dortmund, but the Japanese international refused to celebrate, as did the fans.
The Dortmund players joined the supporters at the full-time whistle as 'You'll Never Walk Alone' was sung by both sets of fans, after the referee decided not to add any injury time.
Very sad to hear a Dortmund fan passed away during the game against Mainz. BvB fans reacted by singing YNWA.https://t.co/BFI1K7jTgW— TheFootballRepublic (@TheFootballRep) 13 March 2016
Reus said the players had been confused by what was happening: "On the pitch, we had no idea what was going on and were initially irritated by the silence.
"Our coach Thomas Tuchel came to us and explained what had happened. It obviously put a shadow over the game, and compliment to the fans for the way they behaved."
Borussia Dortmund president Reinhard Rauball told Sky Sports: "In the name of Borussia Dortmund, I wish to express my heartfelt sympathy and condolences to the families of those affected. Our thoughts are with them.
"We were informed pretty quickly in the stands of what had happened [and] from that moment on, the game fell right into the background. I could no longer concentrate on it.
"The fans' reaction was spontaneous. I've never experienced anything like it. I can only pay a huge compliment to all of the fans for the way in which they expressed their sorrow and respect in this way, in a unified manner.
"I take my hat off to the Mainz fans who, without exception, joined us in mourning and stopped supporting their team.
"The scarves held in the air when 'You'll Never Walk Alone' was sung has my utmost respect. It shows how deeply rooted honour and respect of each other exists and can be expressed in such a short period of time, and I can only take my hat off to that."
As the game neared its end, the club tweeted: "There are more important things than football. More info to follow. #RIP."
Es gibt Wichtigeres als Fußball. Mehr Infos, wenn wir sie haben. // There are more important things than football. More info to follow. #RIP— Borussia Dortmund (@BVB) 13 March 2016
They then added: "You'll never walk alone."
After the final whistle, Dortmund released a statement confirming the tragedy.
"During the game there were two tragic incidents in the stadium, in which two fans had to be resuscitated," the statement read.
"Unfortunately one person didn't make it and has died. We are full of sadness and our thoughts are with their families."
The statement added that the second fan appeared to be in stable condition in hospital.