A pretensioner mechanism for a vehicle seat belt comprises a loading device for applying a preload to the seat belt by pulling a segment of the belt in a direction transverse to a path along which is normally extends. The loading device includes a pulling member having a portion engageable with the belt segment and rotatable about an axis spaced apart from and oriented transversely to said normal path of the belt segment. A one-way clamp engages the belt and substantially prevents release of the applied preload. An interlocking device between the loading device and the clamp operates the clamp in response to movement of the pulling member in a direction opposite to the direction in which it moves to apply the preload to the belt.