Locke was guilty of a bad pass back after 70 minutes which presented a Belarus substitute, Nikolai Rynduk, with the chance to convert the winner in the Dinamo Stadium, where the Scots' senior side will play today's World Cup qualifier.
The goal came after Scotland had survived some heavy first-half pressure. Tommy Craig's side are already out of the qualification race and yesterday he gave Under-21 debuts to three players: Kevin James of Falkirk, Steve Renicks of Hamilton and Lee Peacock of Carlisle.
James found himself heavily involved in the first-half action but he was powerless to prevent the first attack by Belarus in only the third minute. The dangerous Vladimir Makovski swept in a shot from inside the box and Scotland's goalkeeper, Roddy McKenzie, was grateful to see the ball rebound into his arms off the inside of a post.
Makovski soon combined with his twin brother, Mikhail, who forced McKenzie into another save. Scotland then had the Motherwell midfielder Steve McMillan booked after 27 minutes for a bad foul on Vladimir Ostrikov.
After Renicks had required treatment, the St Mirren striker Ricky Gillies also had to be carried off after suffering a knee injury. Gillies received attention for a couple of minutes, but it became clear that he could not continue and Kilmarnock's Alex Burke came on as a 39th-minute substitute.
Belarus, backed by an enthusiastic crowd taking advantage of free entry, claimed a penalty just before half-time when Vladimir Makovski thought he had been body checked by Darren Dodds, but the Polish referee was unimpressed. He took a stricter view shortly after half-time when Burke was guilty of a heavy foul, and the Scottish substitute collected a yellow card.
Craig made a couple of substitutions as he withdrew Peacock and Renicks to send on Gary Teale of Clydebank and Martin O'Neill of Clyde. But Belarus promptly snatched the lead when Locke's pass back was too short for his Hearts' team-mate, McKenzie. Rynduk nipped in to stab the ball under the keeper for the game's only goal.
Belarus Under-21: Gaev; Ostrikov, Tchaifa, Khraprovski, Loukashevitch (Volodenkov, 56), M Makovski, Razovmov, Kashkar, Skripchenko, V Makovski (Rynduk, h-t), Romashchenko (Lisovski, 75).
Scotland Under-21: McKenzie (Hearts); Renicks (Hamilton), Davidson (St Johnstone), Locke (Hearts), James (Falkirk), Dods (Hibernian), Bagen (Kilmarnock), Rowson (Aberdeen), Peacock (Carlisle), Gillies (St Mirren), McMillan (Motherwell). Substitutes: Burke (Kilmarnock) for Gillies, 30; Teale (Clydebank) for Peacock, 62; O'Neill (Clyde) for Renicks, 63.
Referee: Y Granat (Poland).Reuse content