The United Nations Security Council (UNSC) has voted on a resolution authorising a no-fly zone over Libya and "all necessary measures" - code for military action - to protect civilians.
Ten of the council's 15 members voted in favour of the resolution on Thursday, while Russia, China, Germany, India and Brazil abstained.
Despite choosing not to use its veto to block the resolution, China on Friday expressed serious concerns about imposing the no-fly zone.
But no votes were recorded against the resolution, which was co-sponsored by France, Britain, Lebanon and the United States.
Under the no-fly zone, only military aircraft are forbidden to fly in Libyan airspace. It exempts commercial and humanitarian flights.
In Benghazi, the main opposition stronghold, a large crowd watching the vote on an outdoor TV projection burst into celebration and green and red fireworks filled the air.
Reporting from Benghazi, Al Jazeera's Tony Birtley said the city had been given new buoyancy. "They can go back into this fight with renewed vigour, with renewed hope," he said.
Reporting from the eastern city of Tobruk, Al Jazeera's James Bays said the sound of gunfire rang out through the night on Thursday. "Guns were not being fired in anger, but in celebration," he said.
However, Bays added that it was "worth injecting a note of caution" into the current excitement, as Gaddafi's troops were still standing firm.
The resolution came just a few hours after Muammar Gaddafi, the embattled Libyan leader, warned residents of Benghazi that his forces would show "no mercy" in an impending assault on the city.
"The matter has been decided ... we are coming," he said in a radio address on Thursday.
The Libyan leader called pro-democracy fighters in Benghazi "armed gangsters" and urged residents to attack them, saying: "You all go out and cleanse the city of Benghazi.
In an interview broadcast just before the Security Council voted on the resolution, Gaddafi dismissed the body's actions.
"The UN Security Council has no mandate. We don't acknowledge their resolutions," he told the Portuguese public Radiotelevisao Portuguesa.
He pledged to respond harshly to UN-sponsored attacks. "If the world is crazy, we will be crazy too," he said.
Speaking to reporters in Tripoli after the vote, Khalid Kaim, the Libyan deputy foreign minister, took a conciliatory tone, offering to negotiate a ceasefire with the rebels.
"We are ready for this decision (a ceasefire) but we require an interlocutor to discuss how to implement it," Kaim told a news conference.
"We discussed last night with the UN envoy (for Libya, Jordan's Abdul Ilah Khatib) and asked legitimate questions on the application of a ceasefire," he said.
Kaim indicated that Libya would "react positively to the UN resolution, and we will prove this willingness while guaranteeing protection to civilians."
Diplomats indicated that air strikes from a coalition led by Britain, France and the United States could be imminent; however, the UN resolution rules out sending foreign ground troops.
Susan Rice, the US ambassador to the UN, said "This resolution demands an immediate ceasefire and a complete end to violence and attacks against civilians,
"The security council has authorised the use of force, including enforcement of a no-fly zone to protect civilians and civilian areas targeted by Colonel Gaddafi, his intelligence and security forces and his mercenaries," Rice said.
Earlier the Libyan defence ministry warned that "any military operation against Libya will expose all air and maritime traffic in the Mediterranean to danger."
