American and European forces began a broad campaign of strikes against the government of Col. Muammar el-Gaddafi on Saturday, unleashing warplanes and missiles in a military intervention on a scale not seen in the Arab world since the Iraq war.

The mission to impose a United Nations-sanctioned no-fly zone and keep Colonel Gaddafi from using air power against beleaguered rebel forces was portrayed by Pentagon and NATO officials as under French and British leadership.

But the Pentagon said that American forces were mounting an initial campaign to knock out Libya's air-defense systems, firing volley after volley of Tomahawk missiles from nearby ships against missile, radar and communications centers around Tripoli, the capital, and the western cities of Misurata and Surt.

Early Sunday, the sound of anti-aircraft fire and screaming fighter jets echoed across Tripoli, punctuated by heavy explosions.

Mohamed Zweid, secretary of the Libyan Parliament, called the intervention "a barbaric and armed attack" and said that it had "caused some real harm against civilians and buildings." But he declined to specify which civilian buildings or locations were hit. And other officials took pains to show reporters a group of civilians they portrayed as volunteers who had flocked to Mr. Gaddafi's compound to shield him from bombs.

President Obama, speaking during a visit to Brazil, reiterated promises that no American ground forces would be used.

"I am deeply aware of the risks of any military action, no matter what limits we place on it," he said.

"I want the American people to know that the use of force is not our first choice, and it's not a choice that I make lightly. But we can't stand idly by when a tyrant tells his people that there will be no mercy."

The campaign began with French warplane missions even before the end of an emergency summit meeting in Paris, where leaders, reacting to news that Colonel Gaddafi's forces were attacking the rebel capital city of Benghazi on Saturday morning despite international demands for a cease-fire, said they had no choice but to act to defend Libyan civilians and opposition forces.