"This is not an outcome the US or any of our partners sought," Obama said from Brazil, where he is starting a five-day visit to Latin America. "We cannot stand idly by when a tyrant tells his people there will be no mercy."

Thousands of regime supporters, meanwhile, packed into the sprawling Bab al-Aziziya military camp in the capital, Tripoli, where Gadhafi lives to protect it against attacks.

Explosions rocked the coastal cities, including Tripoli, where anti-aircraft guns could be heard firing overnight.

Libyan TV quoted the armed forces command as saying 64 people were killed and 150 wounded in the allied assault. It said most of the casualties were children.

Operation Odyssey Dawn, as the allied assault has been dubbed, followed an emergency summit in Paris during which 22 leaders and top officials of the countries involved in the operation agreed to do everything necessary to make Gadhafi respect a UN Security Council resolution calling for the no-fly zone and demanding a ceasefire, French President Nicolas Sarkozy said.

US Vice-Admiral William E. Gortney, director of the Pentagon's Joint Staff, told reporters in Washington that US ships and a British submarine had launched the first phase of a missile assault on Libyan air defenses.

Gortney said the mission has two goals: prevent further attacks by Libyan forces on rebels and civilians, and degrade the Libyan military's ability to contest a no-fly zone.

US defense officials cautioned it was too early to fully gauge the impact of the onslaught. But a senior defense official, who spoke on condition of anonymity, said the Americans felt that Libya's air defenses had been heavily damaged given the precision targeting of the cruise missiles.

In Cairo, the Arab League chief said on Sunday that Arabs did not want military strikes by Western powers that hit civilians when the League called for a no-fly zone over Libya.

Secretary-General Amr Moussa also said he was calling for an emergency Arab League meeting to discuss the situation.

"What is happening in Libya differs from the aim of imposing a no-fly zone, and what we want is the protection of civilians and not the bombardment of more civilians," he said.

Some Chinese experts criticized the use of force against a sovereign nation.

Li Qinggong, deputy secretary-general of the China Council for National Security Policy Studies, said the Western powers have resorted to military action after their hopes for political change in Libya, through the uprising, failed to materialize. Now they want to "overthrow the Libyan government", he said.

Michael O'Hanlon, a senior fellow with the Brookings Institution, said it would be especially important to include Arabs and other Muslims to find a solution to the Libyan crisis. He called for a formal joint resolution by the Arab League, the UN, NATO and the African Union, "ideally, stating that its only purpose was defensive and that it would not be used to move on Tripoli or overthrow Gadhafi".