Armed police are on the scene at The Oval after a crossbow bolt was fired into the ground during a cricket match.
Play between Surrey and Middlesex was suspended after the arrow landed in the middle of the pitch.
Fans were evacuated from stands in the central London cricket ground, while players were ordered off the pitch.
There were no reported injuries and police said it is believed the projectile came from outside the ground.
“There have been no arrests,” a Met Police spokesperson said.
“We retain an open mind as to motive.”
A Surrey County Cricket Club spokesman said players and fans were advised to take cover after the arrow appeared.
He said: “All the seating hangs over the concourse, so they’re all under the seating at the moment in the concourse area, which is fully covered.
“We don’t know much more at the moment. We’re not sure if it’s deliberate or accidental or whether it’s come from inside or outside the ground.”