An arrow was fired onto the field of play at The Oval cricket stadium during an English league match on Thursday, forcing the ground to be evacuated and play to be stopped.
London's Metropolitan Police said there are no reported injuries and that no arrests had been made.
The incident "is not being treated as terrorism-related," police said in a statement.
They said they are retaining "an open mind as to (the) motive."
Spectators watching Surrey play Middlesex at the south London venue were urged to take cover after the projectile - which Middlesex described as a "crossbow bolt" on its Twitter feed - landed on the square, near players, at 4.20 p.m. local time (1520 GMT).
Police said it is believed the arrow came from outside the ground.
"Well...we have just officially gone off the field because a metal tipped arrow just landed on the pitch!!" Surrey player Stuart Meaker tweeted.
Police said there was a "controlled evacuation" of the ground.
Play was suspended and later abandoned."It's just a very sad incident," said Richard Gould, Surrey chief executive, "but it could have been far, far worse."