A 10-year-old boy has died and an eight-year-old girl has life-threatening injuries following an incident at a house in Coupar Angus.
Police were called to the Union Street property following a call from a member of the public at 02:30 on Sunday.
A man was found outside with significant injuries then the children were found inside. The girl was taken to hospital. The boy died at the scene.
A 37-year-old man has been arrested in connection with the incident.
The girl is being treated at Ninewells Hospital in Dundee where she is in a critical but stable condition.
Ch Insp Ian Scott said: "At this time, our thoughts are very much with the family and friends of the young boy who has died in this incident and with the young girl who remains in a serious condition in hospital.
"I can confirm that a 37-year-old man has been arrested in connection with this incident.
"Inquiries are ongoing, however, I believe this to have been an isolated and contained incident with no threat to the wider public."
He added: "There will be a police presence at the scene for some time while we undertake our investigation and I would like to thank the local community for their patience whilst we carry out our work.
"There will be additional patrols by local community based officers in Coupar Angus to help provide some reassurance to the community."
He urged anyone who saw or heard anything in the area to contact police.
Perthshire North MSP John Swinney said he was "horrified" by the news of the incident.
"This is devastating news and my heart goes out to the children and their family and friends," he added.
"The Coupar Angus community is so close to home for me and the news is literally impossible to comprehend.
"I urge anyone with relevant information to share that with Police Scotland. I will do all that I can to help everyone affected by this tragedy."