Seems they did beef up security somewhat :
Journalist Andy Mitten, who was at the game, told BBC Radio 5 live: "I was in a section full of Croatia fans. The team was playing really well, everything was good, the people around me were happy but then I noticed a big line of police and soldiers building up in front of the Croatia end. It became clear they knew something was going to happen and it did happen. I was told that you needed a passport to get in the Croatia end, that there had been security over and above the normal.