An arrangement (10) for assembling into batches (Ln), via a phasing device (23), a plurality of products (Pn) traveling on a conveyor belt (16) from an intake zone upstream to an exit zone downstream. The phasing device (23) is placed between the intake zone and the exit zone. The arrangement has at least one retractable catch (60, 62, 64), and the catch (60, 62, 64) is moved successively into an engaged position, causing a first product (Pn) to be slowed down until a second product (Pn) located immediately upstream moves up to it, so as to form a batch (Ln), and into a retracted position which enables the previously formed batch (Ln) to continue its downstream movement at the running speed of the belt.