The atmosphere was building nicely.

Borussia Dortmund and Monaco fans were mingling outside the stadium's North Stand and there was plenty of good-natured, energetic chanting between both sets of supporters.

At this point everything was normal. Everyone was looking forward to a highly-anticipated Champions League quarterfinal. There was absolutely no indication of the worrying scenes that were about to follow.

I was still making my way through the bowels of the stadium to the press box when reports came through of an explosion around 8 kilometers (5 miles) away, at the hotel where the Dortmund players were staying. Some fans in the stadium's famous Südtribüne stand told me they had heard the bang. But details were sketchy at this point: merely that there had been an incident and that kickoff may be delayed.

From bewilderment to shock

As the news filtered through that the incident had directly affected the Borussia Dortmund team, the general mood of bewilderment turned to shock. Worry was etched across the face of fans as they checked their Twitter feeds and Whatsapped friends for information. As soon as it was confirmed that Marc Bartra, Borussia Dortmund's Spanish defender, had sustained an injury, a deep sense of shock began to take hold.

"I can't believe it. We are stunned that this could happen, but thankfully everyone at the stadium is safe. We are all thinking about Marc Bartra right now," one Dortmund fan told me.

Rescheduled game - and hospitality

The stadium announcer told fans that the game would be rescheduled for Wednesday at 18:45 local time (1645 UTC) and fans filed in a subdued manner out of the stadium, diligently following instructions to clear the area. The stadium went from three quarters full to almost completely empty in a little over 20 minutes. The only fans who took a little longer to disperse were those from Monaco, who were chanting Dortmund's name, a gesture which went down very well and underlined the respect between these two clubs. Dortmund fans were even offering stranded Monaco fans their spare rooms for the night, a campaign that picked up speed courtesy of social media hashtag #bedforawayfans.

The police outside the stadium confirmed to me that this was a low-risk game. While it was a high-profile match that was set to capture a great deal of attention, that was purely for sporting reasons. There was a sense that even the police were taken by surprise by this attack, which Borussia Dortmund subsequently confirmed was a targeted attempt to harm the team's players and staff. This is not a game where there is any kind of intense feeling between fans or a historical rivalry, so policing was relatively low-level. But after the attack, helicopters patrolled the area, police sirens could be heard all over the city, and the security presence was stepped up significantly.

Back inside the stadium as night fell, only the media and security staff remained. There was an eerie hush around the stadium at a time when the place should have been rocking.

At times like this the game takes a back seat and the football community comes together to stand against such heinous acts. Somehow the Dortmund players and fans will try to get through this game on Wednesday night, with the memories of this shocking attack still very fresh in their minds.