The Royal Navy has been involved in a stand-off with Spanish fisherman in the latest spat in Gibraltar.
A Spanish flotilla made an illegal incursion into British waters to protest about a new reef that they say restricts their fishing rights.
The 38 fishing boats and some pleasure craft were herded back into Spanish waters by Royal Gibraltar Police and the navy.
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Gibraltar stand-off
The reef is at the centre of a diplomatic row between Britain and Spain, which has seen Madrid introduce additional checks at the border in protest, leaving workers and tourists facing queues of up to five hours to get through
Chief Inspector Castle Yates, of the Royal Gibraltar Police, said the boats met in Spanish waters and, despite efforts by British boats to stop them, crossed into Gibraltan waters before being "pushed" out again.
"At around 9am about 38 Spanish fishing boats and seven or eight pleasure craft converged in the area of the western anchorage," he said.
"We had our own police cordon along with Royal Navy and other assets and we corralled them in the area of the south mole.
"They tried to breach the cordon several times but they were not successful."
He said the boats left Gibraltan waters at around 11am.
He added that police had been aware of the planned protest since Friday and it had passed peacefully, with no arrests.
The floating protest was also met by Spanish Guardia Civil boats, which warned them not to sail too close to the British territory's reef.
The Spaniards, who set out from the Campo de Gibraltar in the country's south, claim the reef restricts their right to fish.
Spain believes Gibraltar's dropping of concrete blocks to the sea floor creating an artificial reef was done to disrupt their fishing fleet.
Gibraltar says it was necessary to protect local fish stocks.
A diplomatic spat between Britain and Spain erupted when the Spaniards introduced additional checks at the border, suggesting that a 50 euro (£43.30) fee could be imposed on every vehicle entering or leaving Gibraltar through its fenced border with Spain.
On Friday Prime Minister David Cameron raised the imposition of the extra checks with European Commission president Jose Manuel Barroso.
In a telephone call, he underlined Britain's belief that the checks were "politically motivated and disproportionate" and therefore contrary to the EU right of free movement.
Fabian Picardo, Gibraltar's chief minister, thanked the British authorities for their help on Sunday.
Picardo, who has reportedly received death threats and been targeted by Spanish internet trolls, wrote on Twitter: "Big thank you also to Royal Navy, Gib Defence Police, HM Customs and Port Authority for their deployment too.
"Cool, professional and calm!"