At about 12.10pm a girl was walking past Lewsey Park in Pastures Way when she was approached by a man, who grabbed her arm and attempted to take her mobile.
The victim managed to get away without anything being stolen.
The offender is described as a large, white man in his late 30s, six feet tall, and with receding grey hair.
At the time of the offence he was seen wearing a white shirt with a blue tie, a black bomber jacket and black trousers.
PC Antony Frezza, investigating, said: “This incident happened in broad daylight and we are keen to speak to anyone who may have witnessed it.
“I would like to encourage anyone who has any information about the offender to get in touch.”
Anyone with information is asked to contact the police on 101 quoting reference number C/15609/2018.