We describe the design and operation of the first stage of the EAS‐UAP extensive air shower array, as a detector of very high energy cosmic rays (1016>Eo> 1014eV). The array is located at the Campus of Puebla University. It consist of 18 liquid scintillator detectors, with an active surface of 1m2each and a detector spacing of 20 m in a square grid. One Auger Water Cherenkov detector is also included as part of the array. In this report we discuss the stability, the calibration, the arrival direction and lateral distribution function reconstruction capabilities of the detector array, as derived from the 10 detectors in operation in the first stage. Our results shows that the angular accuracy in arrival direction is less than 5.5° in the range from 20° to 60°. The measurements in the Water Cherenkov Detector show us the possibility to separete electromagnetic and muon component. The main characteristics of the array allow us also to use it as educational and training facility. © 2003 American Institute of Physics