Yahoo Sports Fantasy Football fumbled two days before the NFL season kicks off, upsetting users and fans trying to make their last-minute picks.

The company's site blocked draft picks, just as fantasy football frenzy peaked this week. An internal server problem prevented users from choosing their picks, freezing some out entirely.

Yahoo Sports spokesperson Nicol Addison said the problem lasted about 90 minutes Tuesday night and has been corrected. She could not immediately identify the nature of the problem but said the site handled double the capacity afterward.

But 90 minutes was long enough to draw plenty of fire. The Yahoo fantasy football discussion board is littered with irate posts containing subject lines like, "Yahoo sucks," and "Very, very, very angry."

"My league is at 35 minutes to draft and nobody can enter," one user said on the message board, complaining that it was the second night in a row that he experienced the problem.

Yahoo Fantasy football allows users to build a roster of professional football players through drafts, trades, and free-agent signings. Teams are measured by on-field player statistics. The industry generates gambling revenue, both at legitimate casinos and in unofficial betting circles created among friends, acquaintances, and co-workers.

Fantasy sports researcher Kim Beason estimates that fantasy players number up to 19.5 million in the United States, Canada, and U.S. protectorates, and among Americans abroad. The industry draws up to $3 billion in annual revenue. Yahoo claims about 4 million players.

"By far, fantasy football is the stalwart, the anchor of the industry," said Beason, an associate professor of parks and recreation management at the University of Mississippi, who has studied fantasy sports for six years.

Beason, who has worked with the Fantasy Sports Trade Association, said during an interview Thursday that fantasy football players from North America number about 13 million and draw about $1.5 billion in annual revenue.

"That's everything from league entry fees that don't funnel back into the market, to sponsorships," he said, adding that his numbers are conservative. "We don't have exact numbers because the industry has just blown up over the past few years. Each year, we have new segments that are joining the markets."

A study for the fantasy sports industry showed that Yahoo is the largest single-source provider for fantasy sports to draw earnings from a free format. That means users can play for free, but they're frequently lured into a paid system that makes them eligible for prizes.