Four England fans spent a day in police custody after scaling a fence to watch an England training session on Friday, raising questions about security measures at Euro 2004.
Bob Mardle, his 19-year-old son Luke and two other fans stacked two tyres together, grabbed a tree branch and hoisted themselves into Benfica's former stadium in Lisbon.
They managed to take a few photographs before being apprehended by police.
"When the police came on the scene they were shocked. They didn't know what to do. They clearly weren't prepared for 'what if'," Mardle senior said.
"We're nobodies, but we could have been four somebodies. We proved yesterday that if someone really wanted to do something, they could," Luke agreed.
However the father and son from London had nothing but praise for the Portuguese police who steered clear of handcuffing them or locking them in a cell. They even had a three-course lunch in the police canteen.
"We had ruby-red, succulent strawberries and the police were joking that we would jump over the fence tomorrow to get more of their good food," Mardle senior said.
They appeared before a judge in the afternoon and were let off with a "lecture".
"I think they (the authorities) were a bit embarrassed. We could have said we were just testing your security and it's crap," Mardle senior said.
A police spokeswoman said she had no knowledge of the particular incident, saying only: "If they were arrested it was because there was security. Someone was there to do their job."
The three-week tournament, which features 16 teams playing in eight cities, kicks off later on Saturday with hosts Portugal playing Greece in Porto. It ends on July 4 with the final in Lisbon.