Sport participation has increased in recent years among the European population, as well as in the city of Zaragoza. Measurement of Physical Activity (PA) practiced by citizens has become an element of interest to guide sport policies in the field of health. According to the study «Analysis of the habits, demands and sports trends of the Zaragoza population 2015», public places are the most used for physical activity. The objective of this study was to design and validate an instrument to measure this activity at green-open-areas (urban parks) through observational methodology. A valid and reliable ad hoc instrument was used to data gathering, following SOPARC observation protocol, with a total of 1908 participants in the target areas at Parque del Agua in Zaragoza city. Results showed that in the analyzed periods, men (55.6% in June and 65% in November) were the greater of the park. Most observed physical activity was walking, followed by running. Both men and women preferentially used the park for PA during the late-night period and mid-day in spring (55.7% and 26.6% in males and 51.7% and 28.4% in females). They accounted for more people at noon (33.8%) and in the morning (31.2%) in autumn. Data also indicate that climatological conditions and family affairs affect both the type of practice in general in the population and in practice in large urban green areas.