The swimmer who brought the Oxford-Cambridge Boat Race to a temporary halt has been arrested on suspicion of a public order offence, Scotland Yard said.
The annual 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. He is currently in a stable condition.
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.
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".
Karl Hudspith, president of the Oxford University Boat Club, blamed Oldfield 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."