A 14-year-old girl was indecently assaulted as she crossed playing fields.
The teenager was walking alone through Janes Lane recreation ground, Burgess Hill, when she was approached from behind.
She managed to fend of the attacker by pushing him away and screaming. Police want to talk to a man who was walking a dog in the vicinity at the time.
PC Allan Philip said: "The dog-walker is not a suspect and he may have information which could aid our enquiries.
"I am also appealing to other people who would have been around at that time in the morning to come forward, anonymously or otherwise, if they have seen anyone acting strangely in the area."
Details of the attack, which happened between 7.30am and 7.40am on Friday, have just been released.
Police are also warning people to be vigilant after an 18-year-old woman was assaulted by a man in St John's Park, Burgess Hill.
She was walking through the park at 11.45pm on Friday, November 21, when she was pushed her over.
The woman managed to kick out and was able to run away. Police do not believe the incidents are linked.
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