The victims, aged 15 and 16, were approached by three men on the playing fields between Clacton Leisure Centre in Vista Road and the nearby rugby club sometime between 4pm and 5pm on Monday May 2.
The suspects demanded they had over their valuables. They searched the older boy and took his mobile phone and grabbed the second victim s bag.
One of the men had what was believed to be a knife.
All three suspects were described as white, aged in their 20s, and of medium build. They wore dark scarves over their faces.
One was about 5ft 11ins tall and wore a dark jacket and jogging bottoms, the second was about 6ft tall and wore a dark tracksuit, and the third was about 5ft 8ins tall and wore a beige cap, light jacket and blue jogging bottoms.
Witnesses or anyone with information are asked to contact Clacton CID on 101.
Alternatively, they can contact Crimestoppers anonymously on 0800 555111 or www.crimestoppers-uk.org