A crowded Barcelona beach was evacuated on Sunday after a diver found an explosive dive on the seabed just metres offshore.

Barcelona’s busy Sant Sebastia was packed with people enjoying the sun and 30-degree day when an off-duty police diver who was swimming in the area came across the suspected metre-long bomb.

City police evacuated the beach after the discovery early on Sunday afternoon (local time), saying on their Twitter account that “a possible explosive device” had been found by officers.

Police said the device was in three metres of water and about 25 metres offshore.

Spanish media speculated that it might be an unexploded device leftover from World War II or even from Spain’s 1936-1939 civil war, but there was no confirmation of that.

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German broadcaster Deutsche Welle reported officials would not be able to confirm the identity of the device until Monday, when navy bomb disposal experts would arrive to carry out a controlled explosion.

A Civil Guard spokeswoman said the beach would remain closed until the area was made safe.

Sant Sebastia is 1.1 kilometres long, and is just a kilometre from the Spanish city’s famous Gothic quarter.

–with AAP