The swimmer who brought today's Boat Race to a temporary halt has been arrested on suspicion of a public order offence, 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.
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.
Police said a man was being detained on suspicion of a section 5 public order offence, namely behaviour likely to cause harassment, alarm or distress.
A spokesman said: "He is currently in custody at a west London police station while inquiries into the circumstances are carried out."
Oldfield, who studied contemporary urbanism at the London School of Economics, has a website called Elitism Leads to Tyranny, which 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: "To Trenton Oldfiled (sic); my team went through seven months of hell, this was the culmination of our careers and you took it from us."
He also said Mr 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."
It was almost half an hour after the unprecedented disruption that the race was restarted. But the drama continued when a clash of oars led to Oxford crew member Hanno Wienhausen breaking his blade, allowing Cambridge to pull clear.
The contest ended in no presentation ceremony and the Boat Race Company labelled it "possibly the most dramatic in Boat Race history".
It also confirmed that Mr Woods was in a stable condition.
A statement on the Boat Race Company Limited (BRCL) website said: "BRCL can confirm that Alex Woods is in a stable condition. He will continue to be monitored by hospital staff.
"At this time, BRCL's concern is for Alex's well-being. Alex's family are with him and he is receiving the best possible medical care."
Sean Bowden, the Oxford University Boat Club coach, said the bowman's collapse was the product of "the most extraordinary and unfortunate chain of events that have conspired against us to take away a win which I think we looked like we were about to take in the race proper".
Dr Woods must have felt desperate, he suggested.
"Obviously our biggest concern is Alex's welfare and it was good to see that he was conscious and taken off to hospital with good care," he said.
"We rowed ourselves into a very good position and the crew looked in good shape. And we were ready to go and again at the restart we put ourselves in a very good position.
"The clash was obviously just one of those extremely unfortunate things. And the outcome of the crash was a broken blade.
"And I guess you can only imagine the desperation that Alex must have been in with only six crew mates left and that's probably how he ended up pushing himself beyond his limits."
Umpire John Garrett said it was former rower and assistant umpire Sir Matthew Pinsent who spotted the swimmer in the water.
"I'm grateful to Matthew for having spotted the swimmer," he said. "He basically said, 'There's something in the water, there's something in the water'. He thought it was some debris and then we realised that it was actually a swimmer.
"We weren't sure what was going to happen, whether he was going to get out of the way in time and then it was quite clear he was just waiting for the boats to come across him so I had to stop the race and restart."
Mr Garrett also said the rules stated that crews had to "abide by their accidents".
"If something happens in a latter stage of the race and there's a breakage, they have to abide by their accident, unless one of the crews is actually off-station and has caused that accident," he said.
"In my judgment Cambridge were not off their station. In fact, in the immediate run-up to the clash I was warning Oxford so in my view Oxford were off their station.
"The collision took place, Oxford came off worse but Cambridge were in the right position and so I was content to allow the race to continue and for the result to stand."
Mr Garrett dismissed an appeal from Oxford cox Zoe de Toledo for the race to be re-run.
And he revealed the issue of protest swimmers in the River Thames was discussed before last year's Boat Race - but said there had been no prior warning today's event would be disrupted.
Sergeant Chris Tranter, of the Metropolitan Police, said the rowers had nearly decapitated the swimmer.
"They almost took his head off," he said.
Today 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.
The race was subsequently restarted and Cambridge rowed to victory.