The social class that played a key role in overthrowing the monarchy and establishing the Roman Republic was primarily the patricians. In 509 BCE, the last king of Rome, Tarquin the Proud, was overthrown, largely due to a combination of dissatisfaction with his rule and a revolt led by noble families. The patricians, who were the elite class in Roman society, sought to establish a system of governance that would give them more power and representation, leading to the creation of the Roman Republic.
However, it's also important to note that the broader social dynamics involved the plebeians, who were the common people of Rome. After the establishment of the Republic, there was significant class struggle, which eventually led to political reforms that allowed plebeians more rights and representation, including the creation of the office of the Tribune of the Plebs. Thus, the transformation from monarchy to republic was a complex process involving the interplay of various social classes in Rome.