Police are seeking the help of football fans in tackling terrorism and are asking them to be vigilant at matches.
Officers say stadiums could be targeted as extremists seek out crowded places to attack.
Chief Superintendent Nick Aldworth said: "We know from what we've seen on the ground, and also from terrorist rhetoric, that they are interested in attacking crowded places generally, and that would include stadia.
"We want (fans) to think about what they might do in the event of an attack and we want them to be really, really vigilant.
"And not to be inhibited about reporting stuff to us. If it's trivial to you, it might be significant to us. Don't worry about it - just tell us and leave it to us to sort it out."
He said there was no information about a specific threat to football grounds but previous attacks had shown they could be targeted.
The attacks on the Stade de France in Paris in 2015 and on Manchester Arena last year showed such locations could be hit, Mr Aldworth said.
The request for help was issued on the day before the Premier League season kicks off and forms part of efforts to encourage all members of the public to be "counter-terror citizens".
Mr Aldworth warned fans they may see a large security presence at games and other similar events, but added there will also be secret surveillance as well.
He added: "A lot of places will have very overt security like dogs, they will have a policing presence outside, they will have a strong security and stewarding presence on the inside.
"But, importantly, there will be quite a lot of security measures that people won't see. There will be some covert resources."
The Premier League, the English Football League (EFL) and individual clubs have been enhancing security measures and raising awareness of the threat.
Nearly 250,000 stewards have been given a short guide providing key advice to help protect spectators.
The EFL's Security and operations adviser, Bob Eastwood, said: "The safety of all fans is football's number one priority and we all have a role to play in doing everything we can to keep each other safe - police, officials and supporters."