A British man has been arrested in Spain, accused of calling Spanish police "torturers" for their treatment of motorists at the Gibraltar border and encouraging people to demonstrate outside their homes, writes the Press Association.
The man is being held in Madrid after pictures and comments describing law enforcement staff at the frontier in La Linea (pictured) as "torturers and killers" were posted on a website and shared via social media, the country's Guardia Civil said.
It comes amid heightened tensions at the border between the British Overseas Territory and Spain.
The Guardia Civil has reported officers being pelted with stones by motorcyclists held up in hours of delays at the border after stringent checks were introduced during the diplomatic row with Britain and Gibraltar.
The Briton, who has only been identified by the initials EE, was arrested by Guardia Civil officers from Algeciras, near Gibraltar, at a municipal centre in the capital.
He is being held in custody while items found in a rucksack are analysed, a spokesman said.
"The investigation began when agents discovered a web page in which photographs of officers of the Guardia Civil, national police, and the customs agency were published," he said.
"In addition, these comments were reiterated via different social networks and digital media.
"Investigators located the IP address of the computer from where the detained person made the commentaries, situating it at a location in Madrid.
"Establishing the chance to detain this person, he was held in a Madrid municipal social centre, with a rucksack seized - containing information that is being analysed."
The Foreign Office said it was aware of the arrest of a British national.
"We are providing consular assistance," a spokesman said.
There have been reports of increasing violence at the border over the last fortnight as angry commuters face long queues to get to homes in Andalusia.
As well as reports of missiles being thrown Guardia Civil officers, two unions organised a protest last Friday after reports an officer was injured by a Gibraltar-registered car that failed to stop.
The Royal Gibraltar Police said earlier this week that its "investigations thus far suggest that there is no evidence of the level of disorder at the frontier that has been reported in certain quarters" but that it would make changes in the way vehicles queue at the frontier.
Spain imposed strict checks on traffic at the border with Gibraltar in protest at the creation of an artificial reef.
The Spanish say the 74 concrete blocks on the seabed disrupt an area used by its fishing boats, while the Gibraltarians say it was a necessary environmental measure.
The row, which started at the beginning of this month, has gone all the way to the European Commission, which will send a team to assess the legality of the border checks later this month.
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