A woman was raped in a car at a park where large crowds had converged to enjoy the hot weather, police have said.
The 18-year-old was attacked in a grey or silver Mercedes parked near the entrance to Endcliffe Park in Sheffield at about 23:30 BST on Tuesday.
Hundreds of people had earlier gathered in the green space to soak up the sun.
South Yorkshire Police's Supt Simon Verrall said the incident was "particularly upsetting".
The suspect, who left the scene in the car towards Hunter's Bar roundabout, was described as Asian, in his 20s, wearing a white T-shirt and black joggers or jeans, said police.
Supt Verrall said patrols would take place in the park over the Easter bank holiday weekend.
He added: "They are there to make you feel safe and to help people to enjoy Sheffield's outdoor spaces appropriately and safely."
Earlier, a huge clean-up operation got under way to remove large amounts of litter which had been left strewn across the site after Tuesday's crowds left.
The gathering came as the UK enjoyed the hottest March day in half a century and one day after coronavirus lockdown rules were eased.
Since Monday, two households or groups of up to six people have been able to meet outside in England as the "stay at home" Covid restrictions order came to an end.