Air strikes authorised by the United Nations Security Council against government targets in Libya are likely to begin within hours, after the council voted for a no-fly zone over Libya.
The council has also approved the use of "all necessary measures" - code for military action short of an invasion - to protect civilians against leader Muammar Gaddafi's forces.
Libya has responded by closing its air space to all traffic.
Ten of the council's 15-member states voted in favour of the resolution, with Russia, China and Germany among the five states abstaining.
Rebel forces reacted with joy in their stronghold in the eastern city of Benghazi, cheering and letting off fireworks.
French officials say military action, including air strikes, could start against Col Gaddafi's regime in hours.
It is not thought that the United States would be involved in the first strikes, if they happen, but the British and French are likely to get logistical backup from Arab allies.
Diplomats say the resolution includes all military means short of occupation of the country.
French Foreign Minister Alain Juppe, introducing the resolution, said: "In Libya, for a number of weeks the people's will has been shot down ... by Col Gaddafi who is attacking his own people.
"We cannot let these warmongers do this, we cannot abandon civilians."
Germany says it abstained because it sees considerable dangers and risks in taking military action against Col Gaddafi.
The US ambassador to the UN, Susan Rice, said after the vote: "Today's resolution is a powerful response... to the urgent needs on the ground."
The British ambassador to the UN, Sir Mark Lyall Grant, said: "The international community has come together in deploring the actions of the Gaddafi regime and demanding that the regime end this violence against the Libyan people."
He said Britain was "ready to shoulder our responsibility".
One of Col Gaddafi's sons, Saif al-Islam, says Libya isn't afraid of UN military action and the resolution is unfounded.
Gaddafi threatens 'no mercy'
Reports from Libya say the army has promised to begin a truce on Monday to allow rebels to lay down their arms.
On Thursday, addressing the people of Benghazi, the rebels' main stronghold, Col Gaddafi said his troops were "coming tonight" and there would be "no mercy".
He told rebels to go home, adding that "whoever lays down his weapons" would be pardoned.
Rebel leaders replied by saying their forces would stand firm and not be deterred by the threats.
There has since been heavy fighting in the town of town Misrata, regarded as a key strategic target on the road to Benghazi.
Meanwhile, a trickle of Libyan refugees has become a flood after Colonel Gaddafi's troops entered the rebel bastion of Ajdabiyah, a key city that was held by anti-government forces.
At Libya's eastern border crossing with Egypt, the number of Libyan nationals joining the thousands of migrant workers fleeing the country has shot up in the last two days.
Too late, warns McCully
Foreign Affairs Minister Murray McCully has warned the decision to impose a no-fly zone could be too little, too late.
Mr McCully says he welcomes the move but he is concerned it has not come soon enough.
The Minister says it is very significant the resolution is supported by the Arab League.
He says this is recognition by leaders in the region they cannot sit back while a leader of a neighbouring state butchers his own people.
However, Mr McCully says he is heartened the Security Council not only imposed the no fly-zone but also authorised the use of force to protect civilians under threat of attack.