One athletics fan urged organisers on social media to “fill the place” with “cheap door tickets”.
Another said they decided against buying tickets as it would cost his family “more than a holiday” when travel and accommodation for one night was factored in.
Culture Secretary Lisa Nandy said she expected attendances to increase as the championships continued.
“It is true that over the past few days we haven’t sold out the stadiums but we are seeing those numbers rising steadily,” she said.
“We have some amazing events coming up, the men’s 100m final, people like Keely Hodgkinson competing and I think you will see that really rise in the next few days.”
She added she thought organisers have “pretty good job here” over ticket prices with “half of all the tickets for the events here in Birmingham are under £30”.
A Birmingham 2026 spokesperson expressed delight at the ticket sales which “have far surpassed our targets”.
“Friday, Saturday and Sunday evening sessions are approaching sell-out, with several other sessions close to capacity,” they added.
“There are still opportunities to be part of the championships at incredibly good value.
“After a fantastic opening day of competition, we’re looking forward to welcoming many more spectators to Alexander Stadium throughout the week and sharing more memorable moments of world-class athletics.”