Euro 2016 organizers have received no specific terrorist threat to the tournament in France starting Friday but do not rule out staging matches behind closed doors in certain scenarios.
They said Wednesday that not only security issues but also adverse weather could lead to such measures.
"We have no knowledge of concrete threats for any specific match of Euro 2016," chief organizer Jacques Lambert told a news conference at the Stade de France, which will host matches including Friday's tournament opener between France and Romania and the July 10 final.
The Stade de France was one of the locations targeted by terrorists on a night of deadly attacks in Paris on November 13, which left 130 people dead.
Lambert said security measures had been stepped up in the wake of the attacks, but that "we never considered cancelling the fan zones" in the nine host cities.
Lambert said that UEFA has "considered all alternatives," as every organizing committee has to look into exceptional circumstances, and that "among these we may have to organize matches behind closed doors."
But, referring to recent storms in France and a wet match at Euro 2012, he said that weather could also prompt such a move.
Fans face two security checks at matches and fan zones but not all of them will have to go through airport-style screenings.
Lambert insisted "we are vigilant" and "we will react" but said that the tournament itself should suffer as little as possible from the security situation.
He added that "our main aim is to move out of the negative spiral over which we don't have much control" and remain upbeat overall.
"We think the atmosphere will be great," Lambert said.
UEFA interim general secretary Theodore Theodoridis agreed, saying: "We are looking forward to a party ... We are quite impatient and can't wait for Friday and get the football started."
UEFA director Markus Kallen said that 99 per cent of the tickets for the 51 matches are sold, with just 8,000 left; and that Portugal's arrival Thursday would see all teams present in France.
Spectators could even include the banned UEFA president Michel Platini, as Theodoridis said UEFA had received clarification from the FIFA ethics committee on the issue.