Police were called to reports of an assault on a playing field off Colton Road, Whitkirk, at about 7.20pm yesterday (Thursday).
The victim, a 71-year-old man, was taking part in an organised running group, believed to be the St Theresa's Athletic Club, when the attack took place.
A spokesman for West Yorkshire Police said a group of youths blocked the club's path during the session.
When the elderly man tried to intervene, he was assaulted.
The victim suffered facial injuries and was treated at the scene by paramedics.
Two youths have been identified as suspects and are due to be interviewed about the incident, the spokesman added.