Firefighters turned out to a former Leeds printworks after youngsters set cardboard alight close to the site today.
Smoke billowed from the old Polestar Petty premises off Whitehall Road, close to the railway line in Holbeck, at around 5.30pm.
Arthur Rouke, who was helping to clear the site, said that a group of young people set fire to cardboard under a tunnel close by - something which "keeps happening".
Workers had used fire extinguishers themselves and called the fire service to make sure the incident was logged, he said.
The former factory was engulfed in flames during a separate fire last October.
Signs outside the site, which is sealed off and under the guard of dogs, now say that it is being demolished.