The biggest stadium in Europe is Camp Nou in Barcelona. With a capacity of 100,000 seats it is one of the biggest football (soccer) specific stadiums in the world. There have been plans to make it even bigger but due to the lack of funding these plans are on hold. It was first opened in 1957.
The second biggest stadium in Europe is the Wembley Arena in London. Since its reopening it holds up to 90,000 people. Again; it is a football specific stadium and doesn’t have a running track.
The biggest stadium in Europe that isn’t a football specific stadium is the Olympic stadium in Moscow. It has a track for athletics events and a capacity of around 85,000 seats.