DARNA, Libya—Two former Afghan Mujahedeen and a six-year detainee at Guantanamo Bay have stepped to the fore of this city's military campaign, training new recruits for the front and to protect the city from infiltrators loyal to Col. Moammar Gadhafi.

The presence of Islamists like these amid the opposition has raised concerns, among some fellow rebels as well as their Western allies, that the goal of some Libyan fighters in battling Col. Gadhafi is to propagate Islamist extremism.

Abdel...