The group said their buses got to London 2 hours early Sunday and they were ordered to get off and find what shelter they could on a cold rainy night, the Daily Mirror reported. They had signed on because they hoped the unpaid work would lead to paid jobs during the Summer Olympics in London.
Many huddled under London Bridge.
Close Protection UK admitted its planning had been faulty. The company apologized to those stuck out in the cold.
"We are not in the business of exploiting anyone," it said in a statement.
Gail Cartmail of the union UNITE called for an investigation, saying there should be no repeat during the Olympics.
"They were just dumped in London," Cartmail said. "Taking advantage of people desperate to find work is despicable."