A mechanical movement timepiece assembly with a mechanical oscillator is formed by a resonator of the balance-hairspring type, and a device for regulating the oscillation frequency thereof using an auxiliary oscillator equipped with a quartz resonator. The regulating device includes a sensor, suitable for detecting the passage of the resonator via the neutral position thereof, a measuring device suitable for measuring, on the basis of position signals supplied by the sensor, a time drift of the mechanical oscillator relative to the auxiliary oscillator, and a device for applying to the resonator mechanical braking pulses when a certain time drift is observed. For this purpose, the resonator has a braking surface which extends over at least a certain sector having a certain length along the oscillation axis and against which a braking member may press in order to momentarily brake the resonator.