Witnesses said the man, who was not identified, grabbed a knife at a pizza restaurant, leaped on a table, dropped his pants and sliced off his penis, The Sun reported Tuesday.
"There was blood everywhere," one witness said. "Everyone ran out of the place."
Police subdued the man with gas and put the severed organ on ice, the Sun said. They took the man to London's St. Thomas's Hospital, where doctors attempted to reattach the penis. A hospital spokesman would not confirm whether the procedure was successful, The Sun said.
Ambulance crews treated several diners for shock.
Police said they have very little information on the man, with a spokesman telling the newspaper: "We believe he's Polish and 35. We don't know if he has a history of mental illness but he's clearly not a well boy."