Update: Qaddafi Is Defiant As 'Zero Hour' Begins in Libya

They may have escaped aboard a private plane as rebels circled Tripoli

Update: Qaddafi has spoken in a live audio address on Libya state TV, which claims it was not pre-recorded. From Al-Jazeera's live-blog:

State TV has broadcast an audio message from Gaddafi. He called the opposition fighting him "traitors".
"I can't understand how Libya is now in this state... you need to find the people behind this state... behind this tragedy... You need to love Libya... how can you allow those people to play with Libya? Instead of being happy we're sad...

He said he expected the opposition to claim his message was pre-recorded: "They will say this message is recorded ... It's Sunday, the 21st of August. Time is 1:37 Libya time....

Gaddafi congratulated his supporters for repelling an attack by rebel "rats" in the capital Tripoli, and accused French President Nicolas Sarkozy of trying to steal the country's oil. Gaddafi made the remarks in a live audio broadcast over state television early on Sunday, adding the rebels were "bent on the destruction of the Libyan people."

Meanwhile, Libyan rebels said they launched an attack on Tripoli in coordination with NATO, and Associated Press reporters heard unusually heavy gunfire and explosions in the capital. A senior official with Libyan Rebel Council says "zero hour has started, the rebels in Tripoli have risen up."

Previously: Tension have been mounting in Libya today, with rebel forces encircling Tripoli, causing Qaddafi to lose his grip, according to the New York Times. "By Saturday afternoon, the rebels had driven Colonel Qaddafi’s forces out of the strategic oil refinery town of Zawiyah, 30 miles west of Tripoli. After a week of heavy fighting there, residents began to celebrate in the main square." Although, in Tripoli, still a Qaddafi stronghold, government officials "continued to insist that they would fight to the end," the Times wrote that "rumors swept Libya that [Qaddafi] was preparing to flee, if he had not already."

Now the rumors are turning into unconfirmed reports. NBC New Chief Foreign Correspondent Richard Engel tweeted: "BREAKING: Libyan TV reporting Qaddafi has fled with two sons."  He added: "odd no reports of seif having fled, only hanibam was moatasim," in reference to his sons. Other reports on Twitter are that Qaddafi left "aboard a private aircraft with two of his sons." One source also indicated that Al-Jazeera wrote that Qaddafi "reportedly" left for Italy, although there is no mention of this on its Libya liveblog yet (only that a top Qaddafi aide fled to Italy). Reports of Qaddafi's departure are still unconfirmed. More recently, Engel tweeted that "Rebels themselves don't know if reports of qaddafi's departure are true. 'we hope' they say..."

News of the intensity in Libya continues to come in from all news sources. CNN reports that rebels have in fact entered Tripoli, with heavy clashes, intense gunfire, and explosions. (Although Government spokesman Moussa Ibrahim told state television: "All of Tripoli is safe and stable.") Reuters tweeted "Tripoli residents receive text messages urging them to 'go out into streets to eliminate agents with weapons," says resident."

A top rebel commander in the west can NOT confirm Qaddafi has left, according to Engel.

We will continue to update with further information -- and confirmation of these Twitter reports -- as they come in.