Officials in Sirte claim Mo'tassim was taken while trying to escape the city in a car with a family.
If confirmed it will be a plus for the interim government as he was the national security advisor to his father.
This Tripoli resident was clear in his opinion..." We got the little one," he said, "now we want the big one."
But there have been false reports before of the capture of Gaddafi family members.
Meanwhile in the city of Sirte former rebel fighters are optimistic that the Gaddafi stronghold will fall very soon.
During the last 24 -hours, Gaddafi loyalists have been either cornered or captured.
The taking of Sirte is the last major step before the interim government controls the whole of the country.
One of Gaddafi's sons reported to be cornered
Libya's interim government says it believes it has one of Muammer Gaddafi's sons cornered in the centre of the ousted leader's hometown of Sirte.
Fierce fighting continues there, with forces loyal to Gaddafi still managing to hold out against troops of the National Transitional Council.
Mo'tassim Gaddafi, who used to be his father's national security adviser, is said to be holed up in a house.
After weeks of gun battles the NTC troops have taken most of Sirte, pushing the Gaddafi loyalists to two suburbs in the north of the city.
Concern about the civilians caught up in the fighting is growing, but many have managed to flee.
One woman said: "It's only now that we feel any peace. My husband suffered an attack. We had some very very difficult days. Only God is our witness."
The interim government says it believes Muammar Gaddafi himself is in hiding somewhere in the Sahara desert.
SOURCE: Euronews
