The Duke of Cambridge and his two eldest children kicked off an inaugural running event on Father's Day.
Prince William, who is celebrating his 39th birthday, surprised runners at the Queen's Sandringham Estate in Norfolk on Sunday.
The duke, Prince George, seven, and Princess Charlotte, six, took to a stage to start the half-marathon event.
Race director Aaron Murrell said the runners "really enjoyed" seeing them.
Mr Murrell said 1,300 runners took part in the Run Sandringham event, which included a half-marathon, a 5K and a community mile at the royal estate, with the aim of raising "many thousands of pounds for good causes".
"We were really happy that HRH The Duke of Cambridge joined us as our official race starter - along with his two children George and Charlotte," he said.
"The runners really enjoyed seeing them there as our special guests, and there was a fantastic atmosphere within the event village as they joined us on stage to start the half-marathon event."
Runner Brian Cook said it was a "nice surprise, and nice of them to do".
Charities that benefitted included the organisers' choices of the East Anglian Air Ambulance, Break and East Anglia's Children's Hospices, along with causes chosen by the individual participants.
It is understood the duke was not running in the event and was there in a private capacity.
A tweet from the Sandringham Estate said it was "sterling stuff from all the runners... despite the damp weather".
Prince William was born second in line to the throne at St Mary's Hospital, Paddington, on 21 June 1982.
He and the Duchess of Cambridge, Catherine, who he married in 2011, welcomed their first child George in 2013, followed by Charlotte in 2015 and Louis in 2018.