Spanish fishermen are protesting against the creation of an artificial reef in the bay of Gibraltar and claim the British territory's authorities are blocking their access to the waters.
Relations between Spain and Britain are tense amid a diplomatic row over Gibraltar's status and the waters nearby.
Some 30 fishing boats circled where 70 three-tonne concrete blocks were dropped in July to form the artificial reef, Reuters reports.
Spain says the reef is illegal and its creation prompted the Spanish Government to ramp up border checks with Gibraltar.
The extra controls have caused long waits for thousands of tourists and locals and led British Prime Minister David Cameron to call on the European Commission to urgently send a team of monitors.
Britain claims the measures are "politically motivated and disproportionate" while Spain says it will sue Gibraltarian authorities for environmental damage from the reef.