Men and women in various states of undress could be seen plunging into the shallow waters of the River Cherwell in Oxford as dawn broke over the city’s spires.
Jumping off Magdalen Bridge on May 1 is treated as an ancient custom by many students, alongside a Latin Mass sung at daybreak outside Magdalen College.
However leaping into the river is a much more recent addition to the festivities, believed to have started in the 1970s, and has been actively discouraged following a spate of injuries.
About 40 people were hurt during the celebrations in 2005, with some of them taken to hospital.
Thames Valley Police said it was deploying extra officers to prevent disorder breaking out with more than 30 premises extending their licences for the day.
The Magdalen Choir sings every year as dawn breaks on May Day from the college’s Great Tower in a tradition dating back to 1509.