Ronaldo, the leading scorer in men's international football with 128 goals in 204 official matches, will be playing in his sixth Euros, having reached the final on his tournament debut in 2004 and winning the competition in 2016.
The 39-year-old has played in Saudi Arabia since his move there in 2023.
The 6,000 free tickets for Friday's training session were snapped up in a few minutes when they went on sale and some are now being offered in small adds for hundreds of euros.
There were several tickets offered for 400 euros while others were on sale for 800 euros on the German kleinanzeigen small adds website.
Portugal play the Czech Republic in their opening Group F match on June 18 before facing Turkey and Georgia.
