As usual, around a quarter million fans have gathered at the Indianapolis Motor Speedway in the U.S. state of Indiana to watch the race.
The race went under a yellow caution flag almost immediately when two cars collided on the second lap.
When the race resumed, defending champion Dan Wheldon seized the lead. Wheldon was still holding the lead at the 67th lap, when another accident raised the caution flag again.
The drivers will complete 200 laps adding up to 500 miles before the day is through.
The 33 open-wheel cars are racing under clear skies, with temperatures of about 30 degrees Celsius.
The pre-race "pace car" was driven by seven-time Tour de France champion Lance Armstrong, who hinted Saturday that he would like to give auto racing a try.