Children taking part in sex acts, faeces smeared across walls, toilets "trashed" and a teenager threatening to slit a staff member's throat.

These are some of the incidents reported by lifeguards who said the introduction of free swimming for under-17s had plunged Oxford's pools into "chaos".

Oxford City Council started the three-year scheme at its pools in Blackbird Leys, Temple Cowley and Summertown more than two weeks ago, in a bid to improve health among youngsters.

But staff said many children were spending hours upon end in pools unsupervised, leaving lifeguards to deal with rowdy behaviour, swearing and verbal abuse.

In what was described as one of the worst incidents, about 10 boys and girls were found by a female worker involved in "sexual activity" in the female changing rooms at Temple Cowley Pool.

When asked to leave, the children thought to be about 14 were verbally abusive and a male member of staff had to be called to deal with the situation.

In a separate incident at Ferry Sports Centre, in Summertown, police were called after a 17-year-old boy allegedly threatened to slit a staff member's throat after he was asked to leave the pool for "unacceptable behaviour".

A staff member, who asked not to be named, said: "We're having to police the pools and act like social workers. We're not able to do our jobs as lifeguards properly and that means there's a risk of someone drowning."

The worker urged the council to review the system, for which £375,000 has been budgeted, and set down a rule that children must be supervised.

Another staff member said the number of lifeguards had not been increased, despite a rise in the number of children swimming.

She said Temple Cowley, which can hold up to 120 swimmers, had introduced a 50-swimmer limit in its main pool, and Blackbird Leys was allowing only 30 swimmers at a time.

Mary Clarkson, the city council's executive member for leisure, said it was unfair to blame the unruly behaviour of a minority of children on the free entry scheme, and was disappointed staff had not contacted her with concerns.

She said: "It's important to remember that it's been the Easter holidays, and there is a novelty factor. There will be a review as a matter of course and I'm sure new systems can be put in place by the summer, if necessary."

Council spokesman Louisa Dean said swimmer numbers were managed with a 'banding system' and duty managers determined how many could use the pool at one time depending on the type of session and number of staff available.

Thames Valley Police spokesman Tim Wiseman said a teenager was warned for breaching public order after officers attended Ferry Pool.