Cricket fans at the Oval ground have been urged to move inside after what one player described as a "metal tipped arrow" was fired on to the pitch.
Play has been suspended in the County Championship clash between Surrey and Middlesex after a "projectile" was fired into the ground, in Kennington, south London, from outside, according to the competition's official Twitter feed.
It said: "Play suspended at Kia Oval after a 'projectile' was fired into the ground from outside. Fans urged to move indoors."
Police have been called.
Middlesex County Cricket Club tweeted: "Play suspended at the Oval on advice of security. Players have left the field after a crossbow bolt landed on the square."
Surrey cricketer Stuart Meaker tweeted: "Well...we have just officially gone off the field because a metal tipped arrow just landed on the pitch!!"
A Surrey spokesman told the Press Association: "We were about to bowl the next ball and the arrow appeared two pitches to the side of the pitch we're playing on at the moment.
"The umpires took it out of the ground and got the players off as quickly as possible.
"Everybody in the ground has now been asked to take cover, there is nobody out in the stands or out in the open in any form.
"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.
"The police are on their way.
"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."