A woman grabbed a swan, straddled it, and strangled it to death in the middle of public park.
She left its body on the ground, where it was found by police after a passerby alerted them.
They also told the RSPCA when they saw the bird being attacked by a woman in Riverside Park, Chester-le-Street, County Durham, on Thursday evening.
Officers found the swan already dead and spoke to two women in connection with the incident.
The RSPCA has appealed for witnesses who saw the bird being killed.
Swans are protected by law and anyone harming a wild bird could be jailed, the charity said.
RSPCA inspector Trevor Walker said: ‘Police were initially called by a member of the public who saw a woman grab the swan, straddle the bird and wring its neck.
‘Officers attended the scene where they found the adult swan dead – with a soft tissue injury consistent with being strangled – and spoke to two women in connection with the incident.
‘The bird’s body was removed by council workers and we were contacted to investigate.’
He added: ‘This sounds like an extremely upsetting incident and this attack has led to the unnecessary, tragic death of a beautiful bird.
‘I’d like to hear from anybody who was in the park at around 7pm on Thursday evening and who may have seen the incident or the women involved.
‘Anyone with information which could aid our investigation can contact our appeal line on 0300 123 8018 and leave me a message.’
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