Cairo/Tripoli, March 20 (DPA) Libyan leader Muammar Gaddafi vowed to defend his country against "colonial aggression" as Western nations launched their first air strikes against Libyan defences and began enforcing a UN-sanctioned no-fly zone over Libya.

A French fighter jet launched the first attack Saturday evening, destroying a Libyan military vehicle on the ground as British and French planes began patrolling the skies.

US and British ships later fired more than 100 Tomahawk cruise missiles at more than 20 air defences by the western capital Tripoli, Gaddafi's stronghold, and the western city of Misrata.

Gaddafi in a brief audio message carried on Libyan state television said he had armed civilians to defend the country's independence, declaring that he would open arms depots to "all Libyans".

"The Libyan people will fight against this aggression," Gaddafi said.

News of the British and French jets entering Libyan airspace came shortly after the opposition called on the international community to take action to save civilians in Libya.

"The international community is very late in taking action," Mustafa Abdel-Jalil, the leader of the opposition's National Council in Benghazi, told broadcaster Al Jazeera.

Western nations had given Gaddafi an ultimatum on Friday to enforce a ceasefire against anti-government rebels and halt attacks on civilians, or face military action that was approved by the UN Security Council on Thursday and earlier backed by the Arab League.

A Libyan government spokesman said Libya was the victim of a "barbaric aggression" by Western countries and insisted Gaddafi's had imposed a ceasefire against anti-government forces.

The secretary-general of the Libyan Public Congress, Mohamed al-Zawi, said the Western attacks on Tripoli and Misrata "caused real harm against civilians and buildings".

But residents in the rebel stronghold of Benghazi fled eastwards to escape attacks by Gaddafi's forces, which had reportedly continued despite the alleged ceasefire and the enforcement of a no-fly zone.

"Far from introducing the ceasefire (Gaddafi) spoke about, he has actually stepped up the brutality and the attacks that we can all see," British Prime Minister David Cameron said in announcing his own country's participation in the military action.

The US Pentagon said the rocket launches were the "first phase" of a military operation against Gaddafi as attacks on civilians continued. Vice Admiral William Gortney said the US would need 6-12 hours to assess the success of its initial strikes on air defences.

Gaddafi's forces were shelling Benghazi despite the government's announcement Friday of an immediate ceasefire. Arab television network Al-Jazeera reported that French fighter planes had destroyed four Libyan tanks by the city.

The opposition Libyan Youth Movement said on Twitter that it had reports of more shelling on the outskirts of Benghazi, with the number of casualties "increasing by the minute".

Footage on Al Arabiya showed a fighter plane falling from the sky, apparently shot down near Benghazi, to cheers in the background. It was not immediately known when the footage was taken.

The government said its armed forces were under attack west of Benghazi by "Al Qaeda affiliates", the official news agency reported.

Libya announced a ceasefire after the UN Security Council passed a resolution imposing a no-fly zone over the country banning flights in Libya's airspace and authorized "all necessary means" to implement the ban.

It was not clear whether Arab nations that had backed the no-fly zone were offering any military support to the Western-led strikes, but the US took pains to note that it was not alone in acting.

"We did not lead this, we did not engage in unilateral actions in any way, but we strongly support the international community," US Secretary of State Hillary Rodham Clinton said at an emergency summit of world nations in Paris.

In a letter to world leaders read earlier Saturday at a press conference by a government spokesman in Tripoli, Gaddafi said the resolution was void because the UN had no right "to interfere in the internal affairs of the country".

"You have no right. You will regret if you get involved in this, our country. We can never shoot a single bullet at our people, it is Al Qaeda," Gaddafi said in the statement.

"I have all the Libyan people supporting me and they are prepared to die for me," said Gaddafi.