According to Mundo Deportivo, only 45,838 spectators attended Barcelona's Champions League home match against Feyenoord.

Barcelona hosted Feyenoord on Wednesday in the first round of the Champions League league phase, beginning their European campaign this season. The match was scheduled for an earlier time slot, and factors including the weekday schedule and heavy rain affected attendance at the stadium.
Rain was heavy for most of the time before the match and continued for a long period into the first half. The construction of the roof at the new Camp Nou remains unresolved, leaving almost all seats exposed. This made attending the match more inconvenient for many members and fans. Many people chose to stay at home in the face of possible heavy rain, which eventually did arrive.
There were many empty seats at the stadium, and the atmosphere was noticeably different from a typical Champions League night. A total of 45,838 spectators attended to witness Barcelona's European debut. The goals and the continued support from the supporters' section helped energize the venue, although the number of fans in the supporters' section was far lower than the away section occupied by Dutch supporters.