The Israeli military used a 500lb (230kg) bomb – a powerful and indiscriminate weapon that generates a massive blast wave and scatters shrapnel over a wide area – when it attacked a target in a crowded beachfront cafe in Gaza on Monday, evidence seen by the Guardian has revealed.

Experts in international law said the use of such a munition despite the known presence of many unprotected civilians, including children, women and elderly people, was almost certainly unlawful and may constitute a war crime.

Fragments of the weapon from the ruins of al-Baqa cafe photographed by the Guardian have been identified by ordnance experts as parts of an MK-82 general purpose 230kg bomb, a US-made staple of many bombing campaigns in recent decades.