A woman was allegedly touched inappropriately by a man in Union Street on Tuesday 27 April.
Officers say two women were helping a man they didn't know to his feet after he was 'slumped in a doorway.'
When he was given a cup of water, it's alleged he touched one of the women inappropriately.
The women then walked away. It happened at around 12:50 pm, opposite Anytime Fitness.
The man is described as being of dark European appearance, in his 40s, and around 5ft 8.
He was wearing a dark grey zip-up hoodie, grey cargo trousers, and grey boots.
A 40-year-old man has been arrested and remains in custody.
"This was a busy time of day with lots of pedestrians in the area and police would like to speak with anyone who may have seen what happened or who has any information concerning this incident.
"Anyone who may have seen this or who has any information is asked to contact police on 612612 or the independent charity Crimestoppers anonymously on 0800 555 111 or via their Anonymous Online Form at Crimestoppers-uk.org"