Juan Ponce de Leon, who landed on the island with Christopher Columbus in 1493, ruled the territory as its first governor.

He put down a rebellion by the native Tainos, a subgroup of the Arawak Indians.

Police officers on patrol in the historic district of the capital San Juan heard a loud bang at 4.30am local time and found the statue broken in pieces, city police commissioner Colonel Jose Juan Garcia said.

The toppling of the memorial, made of melted steel from British cannons, "sounded like an explosion" he said.

The incident occurred just hours before Spain's King Felipe VI was due to meet Puerto Rico governor Pedro Pierluisi and other officials to celebrate the 500th anniversary of the founding of San Juan.

Two years ago, activists marched through the streets of Old San Juan in support of a US movement to eradicate symbols of oppression and demanded that Spain's legacy in Puerto Rico be erased.

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While some statues have been defaced with graffiti, police said this is the first time such a statue had been knocked over.

The statue was located in the Plaza San Jose, near the second oldest surviving Spanish church in the Americas.

Its construction began in 1532 on land donated by Ponce de Leon and its base was erected on an Indigenous settlement.

The statue featured the Spanish explorer facing south with his left hand on his hip and right finger pointed towards the first settlement he founded.

The ruins still mark the spot of the island's first Spanish capital and are a US National Historic Landmark.

The statue also points in the direction of the nearby San Juan Bautista Cathedral that bears Ponce de Leon's remains and is a popular tourist spot.