Police have issued a warning after a man was spotted performing a sex act on himself near a primary school in Stretford .
Officers were called to the scene, near St Matthews Primary School on Poplar Road on Wednesday.
When officers arrived at the scene, the man had left, police said.
"He has left, shall we say, evidence at the scene, which our SOCO officers have seized", GMP Stretford tweeted.
The evidence will be tested, they added.
No arrests have been made but police are investigating.
They plan to carry out an arrest 'imminently', the post said.
