The seven detainees are members of Spain's far-right Alliance for National Unity party. Among them is the party's president Ricardo Saenz de Ynestrillas.

Moorish castle: Potent symbol

They are being held on suspicion of committing an act likely to breach the peace after the protest at the castle.

The demonstration was held to coincide with the 20th anniversary of the introduction of Spain's current constitution.

"We believe that on the 20th anniversary of our constitution, Gibraltar, which is Spanish, must be returned to us immediately," said Mr Ynestrillas.

Residents gathered and hurled insults at the nationalists on seeing the Spanish flag flying.

The protest took place as less than 20 party members demonstrated on the border with placards against British rule of the rock.

The castle was built in 711 by the Moors. It and is a popular tourist site and symbol of British-ruled Gibraltar.

Gibraltar, situated on Spain's southern tip, has troubled relations between Madrid and London since the land was ceded to Britain under the Treaty of Utrecht in 1713. Spain claims sovereignty over the rock.