A woman who was sexually assaulted while running at night says the "awful" attack caught her "completely off-guard".
Rachel Stride, 21, was running on The Downs in Bristol on Monday at about 19:45 GMT when a man grabbed her from behind.
She has waived her right to anonymity to talk about the attack and appeal for help in catching the offender.
"You always think it'll never be you until it suddenly is," she said.
Ms Stride was assaulted as she approached Ladies Mile from Circular Road. She called police who arrived within 15 minutes with dogs to search the area but the man was not found.
She stayed in the area to warn other runners to avoid using the route she had taken.
Talking about the assault, Ms Stride said: "They kept getting closer and closer so I thought I will get out of the way and then I just felt this really firm grip on my backside and I turned around and I just saw this man completely covered in black, hood up and maybe a mask on. I can't completely remember.
"I did not manage to see his face clearly. I turned around and he just sprinted back up the hill."
She said she regularly runs on The Downs and although she felt scared when she approached unlit areas, she had got used to it, but the attack had changed that.
"To be personally attacked, to have that impact on me. I'm a person where I feel strong enough that I could have dealt with it, but unfortunately you always think it'll never be you until it suddenly is.
"I don't think it's right that you have to change how you live your life just because there are these horrible people about, but unfortunately it has taken being physical attacked to have that impact on me," she said.
The man police wish to trace is described as white and aged 25-35.. He was wearing a black nylon top with white stripes down the sleeves and had his hood raised. He is also believed to have spoken with an English, potentially Bristolian, accent.
Neighbourhood inspector for North Bristol Insp Lorna Dallimore said that since police issued an appeal for information, they had received a number of calls about incidents which are being reviewed.
She said: "We can't say at present if they're linked and we are appealing for information to determine if it is an isolated incident.
"We would like to reassure the community that we take every incident and report of sexual assault seriously and anyone coming forward who may have experienced something similar will be treated seriously."