§1. The Roman Pontiff obtains full and supreme power in the Church by his acceptance of legitimate election together with episcopal consecration. Therefore, a person elected to the supreme pontificate who is marked with episcopal character obtains this power from the moment of acceptance. If the person elected lacks episcopal character, however, he is to be ordained a bishop immediately.
§2. If it happens that the Roman Pontiff resigns his office, it is required for validity that the resignation is made freely and properly manifested but not that it is accepted by anyone.
Source: Code of Canon Law (Vatican.va)