Moamar Gaddafi's four-decade stranglehold on power in Libya is weakening as violence erupts in the capital Tripoli.

Witnesses say warplanes have fired on protesters in Tripoli, where demonstrators have stormed the streets, setting fire to government buildings.

In the country's east, protesters have reportedly taken control of several cities.

There has been a brutal and swift response by security forces, with reports of a massacre in the capital's Tajura and Fashlum districts. A shoot to kill order is also reportedly in place.

But some senior members of Libya's government are deserting the regime, and the country's representative at the United Nations is calling for international intervention to stop the violence.

'Many dead'

A man calling himself a Libyan resident told Al Jazeera television that warplanes and helicopters were bombing areas of Tripoli and there were "many, many dead".

"Every 20 minutes they are bombing," he said.

"Our people are dying. Anyone who moves, even if they are in their car, they will hit you."

A witness also told the BBC that fighter jets were being used against the protesters.

"Fighter jets ... people are going to take people to cemeteries and they started killing them out in the street," he said.

"Then fighter jets are going down, yes. They are firing on people."

There are also reports helicopters carrying African mercenaries have landed in the streets of Tripoli and have begun firing on residents.

The witness told the BBC residents were being shot and killed indiscriminately.

"There are mercenaries coming from all over the world," he said.

"I've seen some white people killing. I've seen some black people killing people all over the streets."

But Mr Gaddafi's son, Saif al-Islam Gaddafi, was quoted on state television as denying the use of warplanes against protesters, saying the armed forces had launched strikes on arms depots outside urban areas.

"The armed forces have bombarded arms depots situated far from populated areas," the broadcaster said.

Massacre in Tripoli

Meanwhile, residents in Tripoli have described "a massacre" in the Tajura and Fashlum districts of the capital, with indiscriminate shooting.

"What happened today in Tajura was a massacre," one resident of the district said.

"Armed men were firing indiscriminately. There are even women among the dead."

The resident said mosque loudspeakers were putting out appeals for help.

Human Rights Watch said at least 235 people had been killed since last Thursday and hospitals were reportedly struggling to cope with the number of casualties.

Libyan authorities are refusing foreign journalists entry to the country and phone connections are down, so it is difficult to verify the reports.

Meanwhile, two Libyan fighter jets have landed in Malta, where government officials say the pilots defected after they were ordered to bomb protesters.

They said the two pilots, both colonels, took off from a base near Tripoli. One of them has requested political asylum.

Gaddafi rumours

As Mr Gaddafi's 42 years of iron rule continued to weaken, the country's deputy foreign minister denied reports the Libyan leader had fled to Venezuela.

"This news is groundless. It has no basis," Khalid Kayem said on state television.

But senior members of Libya's government have started deserting the regime.

Libya's deputy ambassador to the UN condemned what he described as genocide by Mr Gaddafi's forces against his own people.

Ibrahim Omar al-Dabbashi urged the UN to impose a no-fly zone over Tripoli and for intervention to protect the Libyan people.

He also called on the International Criminal Court to investigate what he says are war crimes.

The country's justice minister also quit his post because of the excessive use of violence.

And at least two of the North African nation's main tribes pledged their allegiance to the protesters.

Travel warning

Australia has upgraded its travel warning to Libya, urging Australians not to go there.

Prime Minister Julia Gillard said the Government was canvassing evacuation options for Australians who may require help to leave the country.

"For Australians who are in Libya, and on our advice in terms of the registered number of Australians there are relatively limited numbers - we're talking about 80 Australians - our advice to them is to travel out of Libya if it is safe for them to do so," she said.

The US State Department has ordered all non-essential staff out of Libya.

Offering a blunt assessment of the deteriorating security situation in Libya, the State Department says violent clashes between protesters and security forces could last for several days.

US citizens staying in Libya are advised to exercise extreme caution, limit travel after dark and make preparations for shelter.

Oil price

The world oil price is spiking as the bloodshed worsens in Africa's major oil producer.

BP chief executive Robert Dudley told AM the company was prepared to withdraw its foreign staff from the region.

"We don't have the basis of doing that yet. We haven't ruled out withdrawing, we haven't ruled out staying on either," he said.

"We'll watch it very, very carefully as any business will right now."

- ABC/wires