potrčati

1. to start running; to break into a run
   Dete je potrčalo ka mami. — The child ran off towards his mum.
   Potrčali smo da stignemo autobus. — We started running to catch the bus.
2. (+ za someone) to run after; to dash after
   Potrčao je za lopovom, ali ga nije stigao. — He ran after the thief but didn't catch him.

For the ongoing action, use trčati.