CAIRO – Deep cracks open up in Moammar Gadhafi‘s regime after more than 40 years in power, with diplomats abroad and the justice minister at home resigning, air force pilots defecting and a fire raging at the main government hall after clashes in the capital Tripoli. Protesters called for another night of defiance in Tripoli’s main square despite the government’s heavy crackdown.

Gadhafi’s regime appeared to be preparing a new major assault in the capital Monday night in an attempt to crush unrest that has already swept the eastern parts of the country — leaving Libya‘s second largest city in protesters’ control — and was now overwhelming the capital of 2 million people.

State TV at nightfall Monday announced that the military had “stormed the hideouts of saboteurs” and called on the public to back the security forces as protesters called for a new demonstration in central Green Square and in front of Gadhafi’s Tripoli residence.

Military warplanes were seen swooping low over the city in the evening, and snipers had taken position on the roofs of buildings around Tripoli, apparently to stop people from outside the capital from joining the march, according to Mohammed Abdul-Malek, a London-based opposition activist in touch with residents.

Communications into the capital appeared to have been cut, and mobile phones of residents could not be reached from outside the country. State TV showed images of hundreds of Gadhafi supporters rallying in central Green Square Monday evening, waving pictures of the Libyan leader and palm fronds.

The eruption of turmoil in the capital after six days of protests and bloody clashes in Libya’s eastern cities sharply escalates the challenge to Gadhafi, the Arab world’s longest ruling leader. His security forces have unleashed the bloodiest crackdown of any Arab country against the wave of protests sweeping the region, which toppled the leaders of Egypt and Tunisia. At least 233 people have been killed so far, according to New York-based Human Rights Watch.

The chaos in Libya, an OPEC country that is a significant oil supplier to Europe, was raising international alarm. Oil prices jumped $1.67 to nearly $88 a barrel Monday amid investor concern. European nations were eying an evacuation of their citizens.

British Prime Minister David Cameron, visiting neighboring Egypt, called the Libyan government‘s crackdown “appalling.”

“The regime is using the most vicious forms of repression against people who want to see that country — which is one of the most closed and one of the most autocratic — make progress,” he told reporters in Cairo.

The heaviest fighting so far has been in the east. In Libya’s second largest city, Benghazi, security forces opened fire on Sunday on protesters storming police stations and government buildings. But in several instances, units of the military turned against them and sided with protesters.

By Monday, protesters had claimed control of the city, overrunning its main security headquarters, called the Katiba.

Celebrating protesters raised the flag of the country’s old monarchy, toppled in 1969 by a Gadhafi-led military coup, over Benghazi’s main courthouse and on tanks around the city.

“Gadhafi needs one more push and he is gone,” said Amal Roqaqie, a lawyer at the Benghazi court, saying protesters are “imposing a new reality … Tripoli will be our capital. We are imposing a new order and new state, a civil constitutional and with transitional government.”

Gadhafi’s son, Seif al-Islam, went on state TV in the early hours Monday with a sometimes confused speech of nearly 40 minutes, vowing to fight and warning that if protests continue, a civil war will erupt in which Libya’s oil wealth “will be burned.”

“Moammar Gadhafi, our leader, is leading the battle in Tripoli, and we are with him,” he said. “The armed forces are with him. Tens of thousands are heading here to be with him. We will fight until the last man, the last woman, the last bullet.” he said.

He also promised “historic” reforms in Libya if protests stop, and on Monday state TV said he had formed a commission to investigate deaths during the unrest. Protesters ignored the vague gestures. Even as he spoke, the first clashes between protesters and security forces in the heart of Tripoli were still raging, lasting until dawn.

During the day Monday, a fire raged at the People’s Hall, the main hall for government gatherings where the country’s equivalent of a parliament holds its sessions several times a year, the pro-government news web site Qureyna said.

It also reported the first major sign of discontent in Gadhafi’s government, saying justice minister Mustafa Abdel-Jalil resigned from his post to protest the “excessive use of force against unarmed protesters.”

Libya’s U.N. ambassadors called for Gadhafi to step down, and there were reports of a string of ambassadors abroad defecting. Libya’s former ambassador to the Arab League in Cairo, Abdel-Moneim al-Houni, who a day earlier resigned from his post to side with protesters, issued a statement demanding Gadhafi and his commanders and aides be put on trial for “the mass killings in Libya.”

“Gadhafi’s regime is now in the trash of history because he betrayed his nation and his people,” al-Houni said.

A Libyan diplomat in China, Hussein el-Sadek el-Mesrati, told Al-Jazeera, “I resigned from representing the government of Mussolini and Hitler.”

Two Mirage warplanes from the Libyan airforce fled a Tripoli air base and landed on the nearby island of Malta, and their pilots — two colonels — asked for political asylum, Maltese military officials said.

The capital Tripoli was largely shut down Monday, with schools, government offices and most stores closed, except for a few bakeries serving residents hunkered in their homes, residents said. Outside, armed members of pro-government organizations called “Revolutionary Committees” circulated in the streets hunting for protesters in Tripoli’s old city, said one protester, named Fathi.

Protesters planed new marches Monday evening in the capital’s main Green Square and at the leader’s residence.

A similar march the night before sparked scenes of mayhem in the long heavily secured capital.

Sunday evening, protesters from various parts of the city streamed into Green Square, all but taking over the plaza and surrounding streets in the area between Tripoli’s Ottoman-era old city and its Italian-style downtown. That was when the backlash began, with snipers firing down from rooftops and militiamen attacking the crowds, shooting and chasing people down side streets, according to several witnesses and protests.

Gadhafi supporters in pickup trucks and cars raced through the square, shooting automatic weapons. “They were driving like mad men searching for someone to kill. … It was total chaos, shooting and shouting,” said one 28-year-old protester.

The witnesses reported seeing casualties, but the number could not be confirmed. One witness, named Fathi, said he saw at least two he believed were dead and many more wounded. After midnight, protesters took over the main Tripoli offices of two state-run satellite stations, Al-Jamahiriya-1 and Al-Shebabiya, one witness said.

Fragmentation is a real danger in Libya, a country of deep tribal divisions and a historic rivalry between Tripoli and Benghazi. The system of rule created by Gadhafi — the “Jamahiriya,” or “rule by masses” — is highly decentralized, run by “popular committees” in a complicated hierarchy that effectively means there is no real center of decisionmaking except Gadhafi, his sons and their top aides.

Seif has often been put forward as the regime’s face of reform and is often cited as a likely successor to his father. Seif’s younger brother Mutassim is the national security adviser, with a strong role in the military and security forces, and another brother Khamis heads the army’s 32nd Brigade, which according to U.S. diplomats is the best trained and best equipped force in the military.

The revolt in Benghazi and other cities in the east illustrated the possibility of the country crumbling.

In Benghazi, cars honked their horns in celebration and protesters in the streets chanted “Long live Libya” on Monday after bloody clashes Sunday that killed at least 60 people as security forces defending besieged stations opened fire with heavy caliber machine guns and anti-aircraft guns.

Benghazi’s airport was closed, according to an airport official in Cairo. A Turkish Airlines flight trying to land in Benghazi to evacuate Turkish citizens Monday was turned away, told by ground control to circle over the airport then to return to Istanbul.

There were fears of chaos as young men — including regime supporters — seized weapons from the Katiba and other captured security buildings. “The youths now have arms and that’s worrying,” said Iman, a doctor at the main hospital. “We are appealing to the wise men of every neighborhood to rein in the youths.”

Youth volunteers were directing traffic and guarding homes and public facilities, said Najla, a lawyer and university lecturer in Benghazi. She and other residents said police had disappeared from the streets.

After seizing the Katiba, protesters found the bodies of 13 uniformed security officers inside who had been handcuffed and shot in the head, then set on fire, said Hassan, the doctor. He said protesters believed the 13 had been executed by fellow security forces for refusing to attack protesters.

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AP correspondents Sarah El Deeb and Hamza Hendawi in Cairo contributed to this report.