Jimmy Thoronka, 20, disappeared at the end of the Games and failed to return to the Ebola-hit west African country.
The Guardian newspaper said it had interviewed him before he was detained for overstaying his visa and that he was held at 7pm yesterday.
A Home Office spokeswoman said only: “We cannot comment on individual cases, but there is assistance available for people to return home when they are not entitled to remain in the UK.”
The newspaper reported that when Thoronka, the country’s number one 100m sprinter, left Sierra Leone for Glasgow, some Ebola cases had been confirmed in a few of the villages surrounding Freetown, but the epidemic had not yet taken hold of the capital. The death toll in the country is now more than 3,500 cases.
The runner said: “I was hoping to win a medal for my country. But during the Games I got the terrible news that my uncle had died, probably from Ebola. I couldn’t stop crying. It was difficult to continue with competing but I tried to carry on.”
He is not working illegally, nor claiming benefits or housing, and understands the legal implications of remaining in the UK after his visa has expired, but said his situation is hopeless.
After the Games, he stayed for a while with friends in Leicester - and found out that his entire immediate family had been killed by Ebola.
Later he went to London and began sleeping in parks and on night buses and begging for £1 from passers-by to buy chips. “Some days I get no food at all. I wash in public toilets and sleep in the park,” he said.
“I wake up around 4am and if I’ve got a bus pass I get on the night bus and sleep there until morning. I met a man who sometimes lets me sleep at his house but I have to wait outside for him to come home at 10 or 11pm and I get very cold.
“We have a cold season in Sierra Leone but it is not cold like England. Some days I don’t think I’m going to make it and just feel like killing myself. My dream is to become one of the best sprinters in the world but I don’t see how that can happen now.”
The newspaper said hundreds of people have been in touch to offer support, and a petition has been set up on change.org to support him.