 The tribunes had the power to unilaterally block any action by a Roman magistrate or the decrees passed by the Roman Senate.The institution of the veto, known to the Romans as the "intercessio", was adopted by the Roman Republic in the 6th century BC to enable the tribunes to protect the mandamus interests of the plebeians (common citizenry) from the encroachments of the patricians, who dominated the Senate.A tribune's veto did not prevent the senate from passing a bill but meant that it was denied the force of law.