Multichannel operation has been suggested for vehicular networks in order to support simultaneously safety and non-safety applications. Without awareness of lower layers, safety messages may be generated out of the control channel interval that leads to synchronous contention at the beginning of control channel interval (CCHI). The jamming attacks at this period are difficult to detect as they can be confused with contention collisions. However, the existence of a parameter which is changed by jamming but collisions can signal an attack during the communication. Based on this, we proposed a detection method that can distinguish between packet lost due to normal collisions and due to jamming attacks. An analytical model is also introduced and validated in order to study the efficiency and the precision of the proposed detection method. Obtained numerical results from analytical model and network simulation show a high detection probability under reactive jamming attacks.