Registration for this fall's Chicago Marathon began Tuesday, but problems with the registration provider caused signup to be suspended.
About 15,000 spots remain open, but it will be at least a couple of days before registration resumes, organizers say.
"We are incredibly sorry that runners interested in signing up for the 2013 Bank of America Chicago Marathon experienced such difficulties today," marathon official Carey Pinkowski said in a statement. "We have been working nonstop with our registration provider to find the root of the problem and a solution."
The race -- scheduled for Sunday, Oct. 13 -- can accommodate 45,000 runners.
Some runners began complaining on social media sites Tuesday afternoon about complications with the registration process. Some were unsure if their registration went through. Others were charged multiple times.
Representatives for Active.com said workers are in the process of refunding any improper charges. Runners can confirm their registration status by visiting the Chicago Marathon registration page.
Organizers say they'll give plenty of notice before they reopen registration for the remaining 15,000 slots.
Another update on the status of the remaining registration will be given by 5 p.m. Thursday.