Jen Reid had never marched in a Black Lives Matter protest before she took to the streets of Bristol, England, on June 7. By the end of that angry day, she had clambered up to stand in the place of a 17th-century slave trader, whose bronze statue had been pulled down and dumped in the city’s harbor.

The image of Ms. Reid, her fist clenched, her right arm thrust upward in a gesture of defiance, spread widely on social media. For many, it seemed the perfect replacement for the notorious merchant, Edward Colston — a flesh-and-blood rebuke to the trader’s cruel legacy, which still hangs over modern Bristol.

At dawn on Wednesday morning, Ms. Reid was up there again in the form of a resin-and-steel sculpture by Marc Quinn. A prominent British sculptor known for his provocative works, Mr. Quinn said that when he saw the image of Ms. Reid on the plinth, he sensed the opportunity for an act of guerrilla art.

He got in touch with Ms. Reid and proposed a clandestine project, in which she would pose for a sculpture that he would install where the Colston statue had stood. Working with a team of 10 people, Mr. Quinn swiftly erected the piece without the approval of city authorities. How long it will be allowed to stay there was not clear.

“She created this iconic image,” Mr. Quinn said in an interview. “I’m just amplifying the moment she created.”

He said he viewed the sculpture as a complement to the protests, one that he hoped would provoke debate about “how we commemorate people in statues.” While he said he did not expect city officials to leave the work in place permanently, he hoped they would leave it long enough to prompt a conversation.