"Words thrown like stones can become bullets," Rome's right-wing mayor, Gianni Alemanno, said after the shooting.
Grillo swiftly moved to distance what he describes as a grass-roots political movement from any calls to violence.
"The movement isn't at all violent," Grillo said.
Sunday was supposed to be a hopeful day with a new government, which, only a day earlier, was forged out of two bitter political enemies. Letta's forces, with strong roots in a former Communist party as well as centrist Christian Democrats, and media mogul Silvio Berlusconi's center-right bloc had agreed after days of negotiations to a kind of truce coalition intent on economic, political and electoral reform.
Then the sound of shots pierced the happy chatter in Piazza Colonna, near a busy shopping street shortly just as Letta and his new ministers were taking their oaths at the sumptuous hall of the Quirinal presidential palace, about a kilometer (half mile) away.
state TV each showed a split screen, on one side, the chaos of panicked people fleeing the square; on the other side, smiling ministers taking the oath of office to work for the good of the nation.
"When I heard the first shot, I turned around and saw a man standing there, some 15 meters (50 feet) away from me. He held his arm out and I saw him fire another five, six shots,"
Television cameraman Fanuel Morelli, who was amazed at what appeared to be the man's deliberate calm, said. "He was firing at the second Carabiniere, who was about 4 meters (13 feet) in front of him."
The gunman was immediately wrestled to the ground by police outside Chigi Palace, which houses the premier's office. The new ministers arrived at the premier's office about 90 minutes later, for their first Cabinet meeting, some of them coming by foot as a way to reassure the public the area was safe.