Five May morning revellers defied police and risked serious injury by leaping from Oxford's Magdalen Bridge.

Barriers set up at 3am and a line of 30 police and 30 private guards on either side of the crossing stopped anyone from plunging into the River Cherwell during the traditional celebrations.

But four hours later the security measures were lifted and a student scaled the walls and leapt.

About half an hour later, at 7.30am, four more male students in their 20s jumped in, one after another. One pretended to hurt his leg, but none were injured.

The jumpers ignored warnings about risks of serious injury issued by Oxfordshire Ambulance Service, Thames Valley Police and the city and county councils, who had jointly decided to close the bridge for the first time since 2001. Last year more than 40 jumpers were injured 12 seriously.

Fyn Gunn, 19, a student at Hartpury College, Gloucestershire, who regularly stays with friends in Oxford, was first to jump.

Dressed in top hat and tails, he bowed as he climbed out of the 20in-deep river. The student said he was worried but "had to jump".

Among four Oxford Brookes students who jumped were Justin Parr, 22, and Daniel Knowles, 23, who stripped off his shirt to leap into the water. Two of the group's friends were among those who jumped last year. This time, they looked on from the bridge.

One said: "Our mate broke his leg last year, so we weren't going to do it again."

However, the authorities yesterday claimed blocking off the bridge had been a success overall, but were unable to say how much it had cost to implement or police the closure.

Chief Insp Steph Cook, of Oxford police, said: "All the people who jumped did so after everything had finished. Very few people jumped compared to last year and nobody was injured.

"Those who jumped were irresponsible, but the main thing is that no-one was injured. The whole point of closing the bridge was to stop a repeat of last year. That has worked so we are very pleased."

Spokesmen for Oxford City Council, which paid for security fencing and 30 guards to support police, and the county council said they were pleased the bridge's closure had stopped the majority of people from jumping.

So few people gathered at the bridge after it reopened that the county council lifted a road closure two hours earlier than the expected time of 9am.

Although the authorities were pleased that no-one was injured, some people were critical of the decision to close the bridge, and during the dawn celebrations a small crowd of revellers shouted abuse at officers.

Student Jennie Hydes, of East Oxford, shouted "This is a waste of national resources, a waste of taxpayers' money" in the direction of police as the Magdalen College Choir performed the Hymnus Eucharisticus from the top of the college's tower at 6am.

She said: "It's ridiculous, total nanny-state behaviour."

Elsewhere, rain failed to deter thousands of people turning out to listen to the choir then move on to Radcliffe Square and Broad Street to watch Morris dancing, or spill into restaurants along High Street for May Morning breakfast.

Catherine King took part in the celebrations with daughter Beatrice, six, and son Henry, four.

She said: "We live in Oxford and come every year, it's a great experience."