A group of long-term unemployed jobseekers were bussed into London to work as unpaid stewards during the diamond jubilee celebrations and told to sleep under London Bridge before working on the river pageant.

Up to 30 jobseekers and another 50 people on apprentice wages were taken to London by coach from Bristol, Bath and Plymouth as part of the government's Work Programme.

Two jobseekers, who did not want to be identified in case they lost their benefits, said they had to camp under London Bridge the night before the pageant. They told the Guardian they had to change into security gear in public, had no access to toilets for 24 hours, and were taken to a swampy campsite outside London after working a 14-hour shift in the pouring rain on the banks of the Thames on Sunday.

One young worker said she was on duty between London Bridge and Tower Bridge during the £12m river spectacle of a 1,000-boat flotilla and members of the Royal family sail by . She said that the security firm Close Protection UK, which won a stewarding contract for the jubilee events, gave her a plastic see-through poncho and a high-visibility jacket for protection against the rain.

Close Protection UK confirmed that it was using up to 30 unpaid staff and 50 apprentices, who were paid £2.80 an hour, for the three-day event in London. A spokesman said the unpaid work was a trial for paid roles at the Olympics, which it had also won a contract to staff. Unpaid staff were expected to work two days out of the three-day holiday.

The firm said it had spent considerable resources on training and equipment that stewards could keep and that the experience was voluntary and did not affect jobseekers keeping their benefits.

The woman said that people were picked up at Bristol at 11pm on Saturday and arrived in London at 3am on Sunday. "We all got off the coach and we were stranded on the side of the road for 20 minutes until they came back and told us all to follow them," she said. "We followed them under London Bridge and that's where they told us to camp out for the night … It was raining and freezing."

A 30-year-old steward told the Guardian that the conditions under the bridge were "cold and wet and we were told to get our head down [to sleep]". He said that it was impossible to pitch a tent because of the concrete floor.

The woman said they were woken at 5.30am and supplied with boots, combat trousers and polo shirts. She said: "They had told the ladies we were getting ready in a minibus around the corner and I went to the minibus and they had failed to open it so it was locked. I waited around to find someone to unlock it, and all of the other girls were coming down trying to get ready and no one was bothering to come down to unlock [it], so some of us, including me, were getting undressed in public in the freezing cold and rain." The men are understood to have changed under the bridge.

The female steward said that after the royal pageant, the group travelled by tube to a campsite in Theydon Bois, Essex, where some had to pitch their tents in the dark.

She said: "London was supposed to be a nice experience, but they left us in the rain. They couldn't give a crap … No one is supposed to be treated like that, [working] for free. I don't want to be treated where I have to sleep under a bridge and wait for food." The male steward said: "It was the worst experience I've ever had. I've had many a job, and many a bad job, but this one was the worst."

Both stewards said they were originally told they would be paid. But when they got to the coach on Saturday night, they said, they were told that the work would be unpaid and that if they did not accept it they would not be considered for well-paid work at the Olympics.

Molly Prince, managing director of Close Protection UK, said in a statement: "We take the welfare of our staff and apprentices very seriously indeed.

"The staff travelling to the jubilee are completing their training and being assessed on the job for NVQ Level 2 in spectator safety after having completed all the knowledge requirements in the classroom and some previous work experience. It is essential that they are assessed in a live work environment in order to complete their chosen qualifications.

"The nature of festival and event work is such that we often travel sleeping on coaches through the night with an early morning pre-event start – it is the nature of the business … It's hard work and not for the faint-hearted.

"We had staff travel from several locations and some arrived earlier than others at the meeting point, which I believe was London Bridge, which was why some had to hang around. This is an unfortunate set of circumstances but not lack of care on the part of CPUK."

The company said it had spent up to £220 on sponsoring security training licences for each participant and that boots and combat trousers cost more than £100.

The charity Tomorrow's People, which set up the placements at Close Protection under the work programme, said it would review the situation, but stressed that unpaid work was valuable and made people more employable. Tomorrow's People is one of eight youth charities that were supported in the Guardian and Observer's Christmas appeal last year.

Abi Levitt, director of development services at the charity, said: "We have been unable to verify the accuracy of the situation with either the people on work experience or the business concerned.

"We will undertake a review of the situation as matter of urgency. Tomorrow's People believes strongly in the value of work experience in helping people to build the skills, confidence and CV they need to get and keep a job and we have an exemplary record going back nearly 30 years for our work with the long-term unemployed."