LONDON — More than 5,000 people stunned event organizers last weekend when they answered a call for a stem cell donor for a 5-year-old cancer patient, breaking Britain’s record for such an event.

The patient, Oscar Saxelby-Lee, received a diagnosis in December of T-cell acute lymphoblastic leukemia, a rare and aggressive cancer that causes the bone marrow to release immature white blood cells. The disease afflicts about 650 people a year in Britain, according to the National Heath Service.

Oscar underwent intense chemotherapy treatment for four weeks, but doctors say he needs a stem cell transplant within three months.

Oscar’s cause was taken up by the community and particularly his school, Pitmaston Primary School in Worcester, a city in central England. “We decided we would do whatever it takes to find a donor for Oscar,” Sue Bladen, the school’s business manager, said in a telephone interview.