Background: Public parks are an important resource for the promotion of physical activity (PA). This is the first study in Colombia and the fourth in Latin America to describe the characteristics of park users and their levels of PA using objective measures. Methods: A systematic observation assessed sex, age, and the level of PA of users of 10 parks in an intermediate-size city in Colombia, classified in low (5 parks) and high (5 parks) socioeconomic status (SES). A total of 10 daily observations were conducted, in 5 days of the week during 3 periods: morning, afternoon, and evening. Results: In total, 16,671 observations were completed, recording 46,047 users. A higher number of users per park, per day, were recorded in high SES (1195) versus low SES (647). More men were observed in low-SES than high-SES parks (70.1% vs 54.2%), as well as more children were observed in low-SES than high-SES parks (30.1% vs 15.9%). Older adults in high-SES parks were more frequent (9.5% vs 5.2%). Moderate to vigorous PA was higher in low-SES parks (71.7% vs 63.2%). Conclusions: Low-SES parks need more green spaces, walk/bike trails, and areas for PA. All parks need new programs to increase the number of users and their PA level, considering sex, age group, and period of the week.