ICC spokesman Fadi El Abdallah said the court never received official confirmation from Libya's rebel authorities about the arrest.
The rebel leadership — which had said Seif al-Islam was captured without giving details on where he was held — seemed stunned. A rebel spokesman, Sadeq al-Kabir, had no explanation and could only say, "This could be all lies."
He also said another captured Gadhafi son, Mohammed, had escaped house arrest. Mustafa Abdel-Jalil, head of the rebel National Transitional Council, announced the detention of a third Gadhafi son, al-Saadi, on Monday.
Riding in a white limousine amid a convoy of armored SUVs, Seif al-Islam took reporters on a drive through parts of the city still under the regime's control, including Bab al-Aziziya, saying, "We are going to hit the hottest spots in Tripoli." AP reporters were among the journalists who saw him and went on the tour.
The tour also covered the district around the Rixos hotel and streets full of armed Gadhafi backers, controlled by roadblocks, and into the Gadhafi stronghold neighborhood Bu Slim.
When asked about the ICC's claim that he was arrested by rebels, he told reporters: "The ICC can go to hell," and added "We are going to break the backbone of the rebels."
Rebels said Monday that they controlled most of Tripoli, but they faced pockets of fierce resistance from regime loyalists firing mortars and anti-aircraft guns. Rebel spokesman Mohammed Abdel-Rahman, who was in Tripoli, said the "danger is still there" as long as the longtime Libyan leader remains on the run.
He warned that pro-Gadhafi brigades are positioned on Tripoli's outskirts and could "be in the middle of the city in half an hour."
An hourlong battle also erupted close to the Rixos Hotel on Tuesday morning, according to AP reporters staying there. The hotel and the area around it are under tight regime control, with scores of heavily armed soldiers stationed just outside it.
A new bout of fighting around the Rixos took place in the afternoon, with the AP reporters saying the sound of explosions and heavy machine-gun fire was much closer than during the morning fighting. A few stray bullets hit the hotel, they said.
It was not immediately clear whether the rebel attack was aimed at capturing the hotel.
The rebels have claimed control of much of the rest of the country outside Tripoli, and the city of Sirte, Gadhafi's hometown to the east of Tripoli, was the most important loyalist bastion to remain fully under his control.
Gadhafi's forces fired off a short-range Scud missile Monday near Sirte — the second one fired during the six-month civil war. On Aug. 15, Libyan government forces launched one near Sirte that landed in the desert outside Brega, injuring no one.
A representative from Sirte on the rebels' National Transitional Council told the AP on Tuesday that the situation in the city was extremely volatile because Gadhafi brigades had retreated to the city after fleeing the Brega oil terminal.
"There is no power in Sirte, we are getting in touch with the people inside only through satellite phones," Hassan al-Daroui told the Associated Press in Benghazi.
He said that many people in Sirte had not even heard about the rebel advance into Tripoli and residents had told him that there were heavily guarded checkpoints all over the city and people were too scared to leave their homes.
"We are worried that Gadhafi wants to just kill as many people as he can before his demise," al-Daroui said. "He knows he is finished, now he wants to bring Sirte down with him."
Farther east from Tripoli, the rebels reported territorial gains at the expense of the regime forces. Mohammed al-Rijail, a rebel spokesman in Benghazi, said rebel fighters have advanced to al-Aqaila, some 25 miles (40 kilometers) from the oil port city of Ras Lanouf.
"There was no resistance and no fighting as Gadhafi forces retreated to Ras Lanouf," he said.
The International Organization for Migration, meanwhile, said that a rescue mission to pluck 300 foreign nationals from the Libyan capital has been delayed by fighting. The Geneva-based group says an IOM-chartered ship will remain off the coast of Tripoli "until security conditions have improved and the safety of staff and migrants can be guaranteed."
Associated Press writers Rami al-Shaheibi in Benghazi, Maggie Michael in Cairo and Mike Corder at The Hague, Netherlands, contributed to this report.