The 158th Xchanging Boat Race between Oxford and Cambridge was brought to a halt by a man swimming across the Thames.
The crews were forced to stop between the two and three-mile marker when the man was spotted swimming in the river.
The blades of the Oxford team's oars narrowly missed the swimmer and the race was halted.
The crews were neck and neck when the man brought the four-and-a-quarter mile race to a standstill.
It is not the first time the Boat Race has 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.
In a further twist a rower at the back of the Oxford boat was removed from the vessel after he collapsed at the end of the race.
Medic Alexander Woods, 27, was lifted from the bow.
Umpire John Garrett told the BBC after the race the man in the water was "totally unexpected" and "like the suffragettes all over again, I think."
Cambridge president Dave Nelson described it as a "pretty dramatic race".
Speaking to the BBC on the riverbank afterwards, he said: "It was really to-ing and fro-ing up until the island.
"We would move a seat, they would move a seat and then suddenly there was this yelling about an obstruction going on and then the next thing I know I see a guy's head just in the middle of the two boats and there's something like 10 or 20 boats following us so that guy was in serious strife if the armada of boats was coming steaming through.
"Then I guess there was all the hoo-ha around the restart and then the clash."
He expressed concern for Dr Woods of Pembroke College, who was being treated by the medical crew but was said to be conscious and sitting up.
Oxford cox Zoe de Toledo, 27, could be seen being comforted at the end of the race.
UPDATE: 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" target="_hplink">UPDATE: 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