Network activity of functionally connected cortical neurons contributes to information processing in the brain and plays a key role for brain functions like sensory processing, learning and memory and behavior. In the present study we recorded population calcium signals generated by correlated action potential firing of neuronal populations in the auditory cortex of mice by means of an optical fiber-based approach. Such network calcium transients (NCaTs) were detected both in response to auditory stimulation and spontaneously without sensory stimulation. These spontaneous and sensory-evoked NCaTs lasted for about 1 s. Auditory-evoked NCaTs consisted of an initial fast rise of the calcium concentration reflecting the short-latency spiking response known from electrical recordings, followed by a slow second component. Spontaneous NCaTs consisted of the slow component only. Stimulation with pure tones revealed a narrower tuning of the fast component compared to the slow component. The amplitudes of the latter ones were dependent on the spontaneous activity level suggesting the mutual exclusion of spontaneous and sound-evoked NCaTs. Optogenetic stimulation of layer 5 neurons induced population calcium signals that were virtually identical to the NCaTs suggesting an important role of these neurons in the generation of NCaTs. Simultaneous recordings of epidural field potentials and NCaTs revealed a tight correlation of the calcium signals and the cortical `up states'. Furthermore, auditory stimulation during a `down state' was able to trigger an NCaT associated with an electrical `up state'. Finally, using two-photon imaging we demonstrated that both neurons and the surrounding neuropil contribute equally to the NCaTs. Taken together, the newly found NCaTs represent sensory-evoked population calcium signals reflecting a period of prolonged neuronal activity in local neuronal clusters of the auditory cortex.