Sammy Clarke, 14, and Siobhan Clarke, 15, left school early on Friday to meet up with their 20-year-old friend Kieran Hartley-Anderson, Essex
Officers believe the girls, from the Maldon area of the county, met Mr Hartley-Anderson at a house in Shoeburyness, around 15 miles away on the coast near Southend
The girls met with 20-year-old Kieran Hartley-Anderson at a house in Shoeburyness (PA)
They have not been seen since leaving to go to a local park that night, officers said.
Chief Inspector Jonathan Baldwin said: "We are extremely concerned for the welfare of Sammy, Siobhan and Kieran as none of them has been seen for nearly two days or have been in touch with relatives or friends.
"We would like to hear from anyone who has been in touch with the girls either by social media or by text or telephone calls."
A search is under way in Southend and police have spoken to friends and family of the three, but no one has heard from them, officers said.