Thousands of demonstrators have gathered in Yemen's southern port city of Aden to call for independence from the north.

Security officials, speaking on condition of anonymity because they were not authorized to brief reporters, say they fired tear gas at demonstrators and fired in the air Sunday after they say protesters attempted to enter a government building. Three were injured and rushed to the hospital.

Police and army deployed heavily ahead of the protest.

The protest falls on the 47th anniversary of Yemen's independence from Great Britain, a day that has become a rallying point for southern secessionists.

Once-independent South Yemen was united with the north in 1990. An attempt to break away from the union in 1994 was ruthlessly crushed by the army of former President Ali Abdullah Saleh.