Fighters with a Kurdish separatist militant group that has waged a decades-long insurgency in Turkey began laying down their weapons in a symbolic ceremony on Friday in northern Iraq, the first concrete step toward a promised disarmament as part of a peace process.

The disarmament of the P.K.K., a group that has battled since the 1980s for Kurdish independence, could end a conflict that has killed more than 40,000 people.