LONDON (AFP) - Britain's historic Boat Race was dramatically disrupted Saturday when a wetsuit-clad protester swam across the River Thames, before Cambridge claimed a controversial victory over rivals Oxford.

Trenton Oldfield caused a half-hour hiatus in the race between England's two oldest universities, which is in now its 158th year, in a stunt that will raise security questions ahead of the Olympics and the queen's Diamond Jubilee.

After the race was restarted there was fresh chaos when the two crews clashed and one of Oxford's oars snapped off.

The team's desperate, abortive efforts to recover caused one of their rowers to collapse as they crossed the finish line, although Oxford boat club president Karl Hudspith later said he "is conscious and will hopefully be OK".

Hudspith blamed the protester for Oxford's defeat, saying in a Twitter

message: "My team went through seven months of hell, this was the culmination of our careers and you took it from us."

As Oldfield was pulled out of the water by a lifeboat crew, the 35-year-old appeared to have a broad smile on his face. But he was arrested and was later charged with an offence under the Public Order Act, police said.

His protest will raise questions about security ahead of an eventful summer in the British capital.

While rowing events for the 2012 London Olympics starting in July will be held at a closed course in Windsor, west of the city, a flotilla marking the queen's 60 years on the throne is set to proceed down the Thames in June.

British media said Oldfield was an Australian activist who gave warning of his protest in an online statement titled: "Elitism leads to tyranny."

"I am swimming into the boats in the hope I can stop them from completing the race and proposing the return of surprise tactics," he apparently wrote.

The two boats were about eight minutes into the race, with Oxford about a third of a length ahead, when the man wearing a black wetsuit swam in front of the two boats, and was nearly hit by the oars of the Oxford crew.

Spectator Mike Emerson said he saw the man come through the trees on the south bank of the river and get into the water long before the boats came past.

"He started swimming, he knew what he was doing," the 60-year-old from Cambridge told AFP.

"He drifted down the river and then he waited near a pontoon until the boats came, and then he deliberately swam out toward the boats."

Umpire John Garrett halted the race after reserve umpire and former British Olympic rower Matthew Pinsent spotted what he thought was debris in the water, which then turned out to be 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," Garrett said.

The race restarted around half an hour later but seconds after that the crews clashed and one of the Oxford oars snapped off, leaving them trailing far behind by the end of the race.

There was further drama after the finish when Oxford crew member Alex Woods, 27, collapsed in the boat and had to receive medical attention.

Coach Sean Bowden said: "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."

The traditional post-race presentation ceremony was abandoned.

"I think it's hard to imagine a Boat Race that contained so much to talk about," Pinsent told the BBC.

Umpire Garrett stood by his decision to award the win to Cambridge, saying the rules stated that crews "have to abide by their accidents".

"In the immediate run-up to the clash I was warning Oxford. Cambridge were in the right position so I was content to let the race continue and the result stand," he said.

The last time the Boat Race was restarted was in 2001 when the crews clashed and one of them lost an oar.

Once just an event for undergraduates at Oxford and Cambridge, the Boat Race has become an increasingly international event, with a succession of top-class rowers from around the world eager to take part.