A protester has jumped into the Thames and halted the 158th universities boat race.

In its 183-year history, the annual Boat Race between English universities Oxford and Cambridge has thrown up its fair share of mutinies, sinkings, even near-death experiences.

The 2012 edition, however, may go down as the most dramatic of them all.

An intruder protesting against elitism and privilege brought both boats to a standstill just over halfway through the 6.8km race when he swam into the middle of the River Thames and narrowly avoided being struck by the oars of both crews.

When the contest restarted after a 31-minute delay, Oxford's German rower Dr Hanno Wienhausen lost half of his oar after the crews clashed, leaving Cambridge an almost unchallenged path to a four-and-a-half-length victory over effectively seven oarsmen.

To add to the drama, Oxford bow man Dr Alexander Woods was taken to nearby Charing Cross Hospital after collapsing post-race. Paramedics later said Woods was in a stable condition but would be kept overnight for monitoring. The reason for his collapse remains unknown.

"This really was the product of the most extraordinary and unfortunate chain of events that have conspired against us," Oxford coach Sean Bowden said.

Protester Trenton Oldfield, who studied contemporary urbanism at the London School of Economics, claimed responsibility for the stunt on a blog titled Elitism Leads To Tyranny. On Sunday, Australian Associated Press said Oldfield was Australian.

After being hauled out of the water by police, Oldfield was arrested and taken to a nearby police station where he was later charged with a public order offence and bailed to appear in court on April 23.

"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," Oxford rowing president Karl Hudspith said.

This was the 158th running of one of England's oldest sporting rivalries.

In other eventful Boat Races, five US athletes in the Oxford squad refused to row for coach Dan Topolski or Oxford University Boat Club president Donald Macdonald in 1987 following disagreements over selection and training methods. The Dark Blues had to rely on oarsmen from the reserve team to make up the numbers but still won.

Boats sank in the 1859, 1912, 1951 and 1978 editions. In that 1859 race, which was staged in a gale, Judge Archibald Levin Smith - who couldn't swim - remained alone in the boat until the water reportedly reached his neck before he was rescued.

The combination of freak events in this year's race was played out in front an estimated 250,000 spectators who lined the river bank to watch what was building up to be one of the tightest in years.

Oxford had just inched into the lead approaching the Chiswick Steps, about 4km along the course, when race umpire John Garrett ordered the crews to stop rowing. Garrett was alerted by assistant umpire Matthew Pinsent - a winner of four Olympic rowing gold medals - who spotted Oldfield's head bobbing in the water.

With the two crews heading straight for him, the protester avoided a potential disaster by ducking under the oars of the Oxford crew.

"They almost took his head off," Sergeant Chris Tranter of the Metropolitan Police told The Associated Press at the finishing line.

"I saw a head," Cambridge's American oarsman Steve Dudek said. "I looked over and thought they had lost a guy (out of the Oxford boat)."

Oldfield, in a wetsuit, resurfaced with a big smile.

"I thought it was very British - the swimmer swam out, stopped a classic British institution and we were responsible for rescuing him," Pinsent said.

In utter confusion, the two boats floated idly, with the crews not knowing whether the race would be restarted or abandoned.

Garrett decided to take them back to the halfway stage, on the Hammersmith bend, where the teams would be on level terms. They had to wait for the river to settle before restarting.

Soon after the resumption, Wienhausen was deprived of the lower half of his oar when the boats veered too close to each other. The German maintained his rowing motion, without his paddle actually touching the water.

At the end of the race, Zoe De Toledo - Oxford's first female cox in 12 years - appealed for the race to be rerun because there was too much wash on the course after the postponement. Her appeal was turned down by Garrett, who said Oxford was to blame for the clash and had to bear the consequences.

The trophy presentation was delayed while doctors attended to Woods, a 27-year-old studying medicine and in his 10th year at Oxford.

"I guess you can only imagine the desperation that Alex must have been in ... that's probably how he ended up pushing himself beyond his limits," Bowden said.

Organisers said Woods' family was with him at the hospital.

Cambridge rowing president Dave Nelson said all the events had taken the gloss off the victory.

"With all the hoohah and the restart and the clash it was a pretty dramatic race," Nelson said. "We're more worried about the Oxford guys right now and we'll reflect later on what's gone on."

The fact that Cambridge extended its lead in the overall series to 81-76 was lost in all the drama. The race is believed to have been watched either live or in highlights in about 200 countries.