President Obama announced Saturday that limited U.S. military action had begun in Libya to protect civilians and rebel forces from Col. Muammar al-Qaddafi.
Obama said the United States and its allies had not sought this outcome but that Qaddafi had given the West no choice.
"We cannot stand idly by when a tyrant tells his people there will be no mercy," the president said from Brazil, where he is starting a five-day visit to Latin America. "We must be clear: actions have consequences."
Obama said he is aware of the risks of taking military action but repeated his assurance that the United States will not send ground forces into Libya.
Qaddafi responded by saying he will arm civilians from what he called "colonial, crusader" aggression by Western forces.
"It is now necessary to open the stores and arm all the masses with all types of weapons to defend the independence, unity and honor of Libya," Qaddafi said in a speech broadcast on state television, Reuters reported.
Obama and Qaddafi spoke after the U.S. military launched a missile attack against Libya's air defenses with strikes along the Libyan coast that were launched by Navy vessels in the Mediterranean.
The Pentagon said it fired 110 cruise missiles at 20 targets.
A U.S. official told Fox News that Qaddafi's air defenses have been "severely disabled" but added, "it's too soon to predict what he and his ground forces may do in response to today's strikes."
Rep. Dennis Kucinich, D-Ohio, expressed concern that the president didn't seek a resolution from Congress before authorizing Saturday's air strikes.
"This is a grave matter because such an attack is specifically without the required constitutional authority," he said.
The international intervention in Libya was launched earlier Saturday, as top officials from the United States, Europe and the Arab world meeting in Paris announced immediate military action to protect civilians. Amid combat between Qaddafi's forces and rebel fighters, French warplanes flew over the country, targeting Qaddafi's forces. American ships and aircraft didn't participate in the initial French air missions.
France, Britain and the United States had warned Qaddafi on Friday that they would resort to military means if he ignored the U.N. resolution demanding a cease-fire.
"This is a broad international effort," Secretary of State Hillary Clinton said from Paris on Saturday. "The world will not sit idly by while more innocent civilians are killed. The United States will support our allies and partners as they move to enforce" the resolution.
Fox News' Jennifer Griffin and Justin Fishel and The Associated Press contributed to this report.