As the players had resumed just after tea, an object was picked up, which happened to be a fired arrow, that had come from outside the ground in South London.
The umpires immediately took the players off, whilst police told spectators to go inside the grounds, as a search was carried out.
Met Police said there were no reported injuries and no arrests have been made at this time as enquiries continue.
The Oval will be hosting an international one-day match between England and West Indies at the end of September.
Meanwhile, Essex continued to lead the way at the top of division one with victory over Somerset at Chelmsford.