The National Guard fired on a republican petition at the Champ de Mars on July 17, 1791.
The petitioners wanted Louis XVI removed after his failed escape during the Flight to Varennes. Two days earlier, the National Constituent Assembly had declared that the king would retain his throne under a constitutional monarchy. Republicans gathered at the Champ de Mars in Paris to demand a different outcome, with thousands signing the petition.
Paris mayor Jean Sylvain Bailly declared martial law after two suspicious men were found beneath the altar and were killed by the crowd. Lafayette then arrived with the National Guard to disperse the gathering. When stones were thrown and warning shots failed to clear the area, the troops opened fire.
The exact death toll remains uncertain, with estimates ranging from about a dozen to 50 people. The massacre temporarily weakened the republican movement and damaged the reputations of Bailly and Lafayette. It also exposed a widening split between those seeking a constitutional monarchy and more radical revolutionaries who wanted to depose the king. The event is sometimes confused with the later republican uprising of August 10, 1792, which helped bring down the monarchy.