A French fighter jet fired on Libyan forces Saturday, kicking off allied military action against Moammar Khadafy - who was making a last-ditch effort to crush rebels.
"Our planes are already preventing air attacks," French President Nicholas Sarkozy said in Paris, where he was joined by British and U.S. leaders for an emergency summit.
"The time for action has come," British Prime Minister David Cameron said.
Canada announced that a naval blockade was also in place. U.S. ships were poised to take out Libyan air defenses with missiles.
"The people of Libya must be protected, and in the absence of an immediate end to the violence against civilians our coalition is prepared to act, and to act with urgency," President Obama said in Brazil.
Secretary of State Clinton said the U.S. would support "all necessary measures" to stop Khadafy but would not put troops on the ground.
The mission began with French planes entering Libyan airspace over Benghazi, which was under siege by pro-Khadafy forces.
Hours later, a French defense official said one of its fighter jets had fired on a Libyan military vehicle.
An increasingly desperate Khadafy had warned the West to stay away.
"You will regret it if you dare to intervene in our country," the loony strongman vowed in a letter read by a government spokesman in Tripoli.
State-run media even raised the specter of human shields, claiming hundreds citizens had converged on targets the West could strike.
Despite a cease-fire declared only a day before, pro-Khadafy forces entered Benghazi early Saturday and continued to shell the city.
Rebel leaders accidentally shot down one of their own fighter jets as they tried to repel the government forces, an opposition activist said.
They later said they drove out Khadafy's henchmen and seized four tanks.
There were reports that Zintan in western Libya was being bombarded and tanks were approaching.
"There are tanks heading towards the southern entrance of Zintan, around 20 to 30 tanks, which are hitting the city and residential areas in the south," a witness told Al Arabiya.
In the western city of Misrata, residents said they were shelled again Saturday.
"I am telling you, we are scared and we are alone", one resident said.
After Khadafy threatened terrible retribution against rebels who did not surrender immediately, the U.N. Security Council authorized a no-fly zone over Libya earlier this week.
A Khadafy spokesman on Saturday read letters to President Obama and other world leaders calling the resolution "invalid" and an "injustice."
"Libya is not yours. Libya is for the Libyans," he wrote.
British Prime Minister David Cameron said Khadafy has no one to blame but himself.
"Colonel Khadafy has made this happen. He has lied to the international community, he has promised a ceasefire, he has broken that ceasefire," he said in Paris.
"He continues to brutalize his own people...we have to enforce the will of the United Nations and we cannot allow the slaughter of civilians to continue."