Libya's Al-Hurra Misrata television also said Ibrahim had been caught outside Sirte and that he had been in a car and veiled, adding that it would soon broadcast footage of his capture.
Ibrahim had been the public voice of the Gaddafi regime until NTC fighters overran Tripoli on August 23.
Despite fleeing the capital along with the deposed dictator, he has continued to issue statements through Syrian-based Arrai television from an unknown location, although not as frequently.
On Friday, Ibrahim had appealed for resolve against "agents and traitors", denounced what he called "genocide" by NATO and its "Libyan agents" and criticised the world community for "inaction".
Global police agency Interpol said, meanwhile, the NTC had requested an arrest notice against Saadi, who is believed to be in Niger.
The Libyan authorities, it said in a statement, wanted him "for allegedly misappropriating properties through force and armed intimidation when he headed the Libyan Football Federation".
Saadi, 38, was last seen in Niger and the red notice calls particularly on countries in the region to help locate and arrest him "with a view to returning him to Libya where an arrest warrant for him has been issued", Interpol said.
However, Niger's Prime Minister Brigi Rafini said his country has no plans to send Saadi home to face justice.
"Saadi Gaddafi is in safety, in security in Niamey, in the hands of the Niger government. There's no question of him being extradited to Libya for the moment," Rafini said.
"We need to be sure he will be allowed a fair defence," he said. "Are those conditions in place today? No."
While the fugitive Gaddafi's whereabouts remain unknown, Libya's defence ministry spokesman Ahmed Bani said in Tripoli that his most prominent son, Seif al-Islam, was in Bani Walid and also said Mutassim was in Sirte.
An NTC field commander in Sirte also told AFP Mutassim was in the Mediterranean city, which lies some 360km east of Tripoli.
"Mutassim is inside and he is commanding his forces. They are using heavy guns as well as snipers, which is making it difficult for us."
Equally fierce resistance from loyalists in the desert town of Bani Walid, Gaddafi's other remaining bastion of support, has stalled a final assault by NTC fighters, said commanders, who urged NATO to increase its air support.
In its latest operational update published on Thursday, NATO said its warplanes had struck an ammunition store, a command and control node, a tank, and two ammunition and missile facilities.
-AAP