LONDON (AFP) - A British couple were fined Wednesday after being found guilty of carrying out a sex act at a crowded open-air concert in London's Hyde Park.

Lisanne Beck, 47, and Simon Murphy, 48, were filmed engaging in sexual activity, including oral sex, on a mobile phone during a set by singer-songwriter Paloma Faith in September 2014.

Shocked onlookers alerted security and the couple, from Swansea in south Wales, who had both drunk several cans of cider, were thrown out, London's Old Bailey heard.

Beck, a waitress, claimed she was trying to shake Murphy, who works as a gardener, so he could use the bathroom.

"He had not been to the toilet and I knew that he gets discomfort in his stomach because of a medical condition - IBS (Irritable Bowel Syndrome)," she said.

Murphy told police he was completely unaware of anything improper happening.

But Judge Charles Wide told the couple they needed to be "taught a lesson" for their "revolting behaviour" after a jury found them guilty of outraging public decency.

They were ordered to pay £1,000 (S$2,200) each, plus £1,750 in prosecution costs.