Runner Laura McLean of Toronto said she heard two explosions outside the medical tent.
"There are people who are really, really bloody," McLean said. "They were pulling them into the medical tent."
Cherie Falgoust was waiting for her husband, who was running the race.
"I was expecting my husband any minute," she said. "I don't know what this building is ... it just blew. Just a big bomb, a loud boom, and then glass everywhere. Something hit my head. I don't know what it was. I just ducked."
The Federal Aviation Administration is warning pilots that it has created a no-fly zone over the site of two explosions at the annual Boston marathon.
The agency said in a notice issued about an hour after the explosions that a no-fly zone with a 3.5-mile radius has been created over 811 Boylston St. The zone is limited to flights under 3,000 feet in altitude, which is lower than most airliners would fly except when taking off or landing.
The notice says the no-fly zone is effective immediately, and will remain in effect until further notice. Pilots planning flights were urged to call their local flight service station.