"We're in a state of emergency," a police officer said, noting however that the atmosphere was peaceful.
On the Russian side on the other hand, only about 10,000 fans were expected by UEFA and the City of Basel.
Camping sites in and around the northern Swiss city were taken over by Dutch fans, while some 3,500 filled the Oranje Dorp (Oranje village) to capacity in Bubendorf, south-east of Basel.
A street in the city centre was to be closed to traffic to allow the orange-clad fans to do their usual pre-game parade.
Led by an orange double-decker bus, the fans traditionally march together to the stadium two or three hours before kick-off, performing a well-choreographed dance: sitting down at red lights, chanting "Hup Holland Hup!" ("Let's go, Holland, Let's go!") at orange and setting off again when the traffic light turns green.
Saturday around noon, the bus was already in place, while numerous fans held up banners and posters to show support to Dutch defender Khalid Boulahrouz, following the death in midweek of his prematurely born baby daughter, Anissa.
The Dutch team said it would wear black armbands on Saturday as a mark of respect for the player and his family, and a number of fans' shirts and banners could be seen featuring the baby girl's name.
Street cleaners, in usual orange, also enjoyed their minute of fame, enthusiastically greeted by Dutch fans everywhere they went.
Meanwhile, the Prince of Orange himself, Willem-Alexander, was to watch Saturday night's match from the stands of St Jakob-Park in Basel, proving once again that football brings everyone together, whether royalty or cleaner.