Dozens of Spanish fishing boats sailed to Gibraltar on Sunday to demand that the British outpost remove 70 concrete blocks it has dropped in their fishing grounds.
British naval and Gibraltar police patrols blocked the 38 Spanish boats from entering disputed waters around the concrete reef, which has sparked an angry diplomatic row between London and Madrid.
In the midst of the spat, Britain's helicopter carrier HMS Illustrious visited a Spanish port on Sunday and a frigate was scheduled to dock in Gibraltar on Monday as part of a pre-planned naval exercise.
The Spanish fishermen from the nearby ports of La Linea de la Concepcion and Algeciras set sail on calm waters across the narrow strait, accompanied by a half dozen Spanish police patrols.
The Spanish fleet approached a cordon set up by a total of 14 British naval and Gibraltar vessels, manoeuvring in close quarters but then turning back with no incident reported beyond an exchange of insults.
About 500 people watched the one-hour protest from the shores of Gibraltar, many waving Gibraltar and British flags. Spanish spectators gathered in the port of La Linea de la Concepcion, some with Spanish flags and T-shirts reading "Gibraltar Is Spanish".
"We just want to send a message to Gibraltar," said Leoncio Fernandez, the head of the La Linea fishermen's guild. "All we want is to fish where we have always fished."
Gibraltar police said their main concern was safety. "When you're working at sea in such close quarters, it can be very dangerous," a police spokesman said.
The Gibraltar government says the reef will regenerate marine life and that the few Spanish fishermen who raked there for shellfish did so illegally under Gibraltar law.
Spanish fishermen, however, say the concrete blocks -- dropped into the sea last month without consultation -- have cut them off from rich fishing grounds and lowered their already meagre incomes.
Britain accuses Spain of retaliating over the reef by imposing excessive customs checks at the border to Gibraltar, leading to daily hours-long queues of cars.
British Prime Minister David Cameron described the checks as "politically motivated" and has pressed the European Union to send observers to the border as soon as possible.
It is the latest in a string of diplomatic rows over the self-governing British overseas territory, which measures just 6.8 square kilometres (2.6 square miles) and is home to about 30,000 people.
Spain ceded Gibraltar to Britain in perpetuity in 1713 but has long argued that it should be returned to Spanish sovereignty. London says it will not do so against the wishes of Gibraltarians, who are staunchly pro-British.
With Madrid's prior agreement, British helicopter and commando carrier HMS Illustrious on Sunday stopped at a Spanish naval base in Rota in southern Spain for a technical stop of several hours before leading naval exercises in the Mediterranean.
A British naval frigate, the HMS Westminster, and two support vessels are due to dock in Gibraltar on Monday before taking part in the naval exercise, codenamed Cougar '13, which was planned long before the latest row broke out.
The British ships will visit several ports, carrying out an exercise with the Albanian armed forces before heading through the Red Sea, Indian Ocean and the Gulf for exercises with other British allies.