LONDON — “You look suspicious,” Marina Abramovic said to an older couple standing to the side of a room in the Serpentine Gallery here on Thursday. The couple looked, well, suspicious, as around them people contemplated panels of bright primary colors, or lay on the floor, eyes closed. Ms. Abramovic took them by the hand, gently asked them to close their eyes, and led them away, walking with a slow, measured tread.

It was Day 2 of “512 Hours,” Ms. Abramovic’s first new work since her 2010 retrospective, “The Artist Is Present,” at the Museum of Modern Art in New York, transformed her from a pioneering performance artist to a celebrity. There, she sat motionless, six days a week, seven hours a day, looking straight at whoever sat down opposite her. This time there is no chair. “There is just me,” she said. “And the public. It is insane what I try to do.”

The idea of “512 Hours,” named for the length of time Ms. Abramovic will spend in the gallery over the duration of the exhibition (running through Aug. 25), is both simple and radical. There is nothing in the Serpentine galleries except lockers, where visitors can put their bags and electronic devices. Ms. Abramovic, as well as an assistant, Lynsey Peisinger, and several museum guards are there. What will happen in the space no one quite knows. “I honestly don’t know; I don’t have a plan,” she said in an interview at the house she is sharing with her assistants during the London show. “That is the point. The idea is that the public are my material, and I am theirs. I will open the gallery myself in the morning and close it at 6 p.m. with my key. I want to understand how I can be in the present moment, be with the public.” On Wednesday, hundreds of people lined up outside to enter the gallery, although on Thursday there was no wait.