More than 112 Tomahawk cruise missiles struck over 20 targets inside Libya today in the opening phase of an international military operation the Pentagon said was aimed at stopping attacks led by Libyan leader Moammar Gadhafi and enforcing a U.N.-backed no-fly zone. President Obama, speaking from Brazil shortly after he authorized the missile attacks, said they were part of a “limited military action” to protect the Libyan people.

“I want the American people to know that the use of force is not our first choice and it’s not a choice I make lightly,” Obama said. “But we cannot stand idly by when a tyrant tells his people that there will be no mercy.”

The first air strikes, in what is being called Operation Odyssey Dawn, were launched from a mix of U.S. surface ships and one British submarine in the Mediterranean Sea at 2 p.m. ET, Vice Adm. William E. Gortney told reporters at a Pentagon briefing.

They targeted Libyan air defense missile sites, early warning radar and key communications facilities around Tripoli, Misratah, and Surt, but no areas east of that or near Benghazi. Because of darkness over Libya, Gortney said it was too early to determine the strikes’ effectiveness.