Student Mabert Quek, 15, was on his way home in Geylang with friends when they realised their usual route had been cordoned off.
Initially, he thought that there was a hostage situation. But after asking around, he realised that an artillery shell was found among the rubble of a hotel undergoing demolition works, about 30m away from his house.
A police spokesman said the relic was discovered at around 12.40pm at the junction of Lorong 22 Geylang and Guillemard Road.
That section was cordoned off while the shell was being removed, causing a traffic jam..