A 72-year-old cyclist is in hospital in a critical condition after she collided with a police quad bike in Melbourne's north-east.

Police officers were on the Darebin Creek Trail in Heidelberg West to target the illegal riding of motorbikes, dirt bikes and monkey bikes when the crash happened about 3.45pm on Thursday.

Police say the woman lost control of her bike when she came around a bend, saw the quad bike and tried to stop.

The St Kilda woman slid and hit the front of the quad bike.

She was taken to the Royal Melbourne hospital in a critical condition.

A police spokesman said the quad bike was being driven at a low speed and that the crash would be investigated.