The swimmer in the Thames who brought today's Boat Race to a temporary halt has been arrested on suspicion of a public order offence and is being held in custody at a west London police station while inquiries are carried out, Scotland Yard said.
The annual Oxford-Cambridge competition was restarted after the wetsuit-clad man appeared close to the boats in the River Thames in London, narrowly avoiding the blade of an Oxford oar.
Sergeant Chris Tranter, of the Metropolitan Police, said the rowers had nearly decapitated the swimmer.
"They almost took his head off," he said.
Cambridge powered to victory but celebrations were muted while Oxford rower Alexander Woods was taken to Charing Cross Hospital after collapsing in the boat.
Sources named the swimmer who stopped the race for the first time since 2001 as Trenton Oldfield.
He is thought to be an LSE educated graduate protesting against elitism. Oldfield's blog, called Elitism Leads to Tyranny, discusses civil disobedience tactics.
He is also joint co-ordinator of a not-for-profit organisation called This Is Not A Gateway, which "creates platforms for critical projects and ideas related to cities".
According to the website, he has worked for more than a decade in non-governmental organisations specialising in urban renewal, cultural and environmental programmes.
Karl Hudspith, president of the Oxford University Boat Club, blamed him for ruining his crew's big day.
He wrote on Twitter:
He also said injured rower Woods was conscious and "will hopefully be ok", offering thanks for the many messages of support the rower had received.
He added: "I'm proud of everyone in the team and how they rowed. They were a credit to themselves and their university."
Saturday was the first time in the Boat Race's history that the event had been disrupted by a swimmer, organisers said.
But it was not the first time it had been temporarily halted. In 2001 the race was stopped by the umpire just over a minute after the start following repeated warnings to both crews to move apart and then a clash of blades for which Oxford was blamed.
A Met police statement said:
Shortly before 14:30hrs on Saturday, 07 April 2012 police were alerted to a man in the River Thames where the Boat Race was being held.
Officers from the Metropolitan Police Service (MPS) Marine Policing Unit (MPU) attended and took the man (no further details at this time) back to the riverside where he was arrested for a section 5 public order offence. He has been taken into custody at a west London police station.
Enquiries into the circumstances of the incident are ongoing.