“I’m a little shaken by this,” he said.

Mr. Cruz said he saw a man, who was dressed in a baseball cap, T -shirt and jeans, hanging around near the sculpture for about 10 minutes before moving toward it.

The weapon was made of metal with sharp edges and resembled a makeshift banjo, according to the police. It left behind about a dozen deep scratches in addition to the gash. Witnesses said the bull was struck dozens of times, and the sound of the battering was deafening.

Some people scattered. Others seemed to think it was a performance. “While he was hitting the bull, they were trying to take pictures,” Mr. Cruz said. “I said, ‘What are you doing? What if this guy turns on you, hits you over the head, kills you?’ They didn’t pay no mind.”

The bull, the symbol of a surging stock market, has been vandalized before. It was splashed with blue paint twice: in 2008 and in 2017. And in 2010, an artist “yarn-bombed” the “Charging Bull” with crochet, which she described as graffiti but with yarn.

On Sunday morning, there was little police presence near the sculpture, and no efforts were made to prevent people from touching or climbing on the bull. More than a hundred tourists lined up to take photographs.

Many did not notice the gash until others pointed it out.

Paola Vallejo, 27, of Mexico City, said she was there to take social-media worthy photos. For her, the real attraction “is the Instagram picture,” she said.

Carlos Swinger, 55, who was visiting from Raleigh, N.C., said he was disappointed to see the damage.