More than 110 Tomahawk missiles, fired from American and British ships and submarines, hit about 20 Libyan air and missile defence targets, US Navy Vice Adm William Gortney said at a Pentagon briefing.
"Coalition forces have launched ''Operation Odyssey Dawn'' to enforce UN Security Council Resolution 1973, which protects the Libyan people from their ruler (Muammar Gaddafi)," Gortney said.
US military forces are on the leading edge of the coalition operation, taking out Libya''s integrated air and missile defence system. "The ordnance is aimed at radars and anti-aircraft sites around the capital of Tripoli and other facilities along the Mediterranean coast," the Pentagon said in a statement.
The US will conduct a damage assessment of the sites, which include SA-5 missiles and communications facilities.
Earlier, French warplanes reportedly have hit four tanks used by the Gaddafi''s forces on the outskirts of the opposition stronghold of Benghazi, on a day when opposition fighters in the city came under constant artillery and mortar fire.
However, Libyan state television reported civilian targets in Tripoli had been bombarded, as well as fuel stores in Misurata. The state news agency reported that there had been "civilian casualties as a result of this aggression".
The action by coalition forces came after their leaders approved military strikes against Gaddafi's forces.
"Today I authorised the Armed Forces of the United States to begin a limited military action in Libya in support of an international effort to protect Libyan civilians. That action has now begun," US President Barack Obama said in Brasilia.
In London, UK Prime Minister David Cameron said British forces were in action over Libya. "What we are doing is necessary, it is legal and it is right... I believe we should not stand aside while this dictator murders his own people." French President Nicolas Sarkozy, earlier, noted his country''s warplanes are already targeting Gaddafi's forces.
Meanwhile, in a brief statement on state television, Gaddafi said the air strikes marked the beginning of another "crusade", adding that the Mediterranean and North Africa were being turned into a "battleground".
He said that arms depots should be opened so that Libyans may defend themselves, Al Jazeera reported.
While, Mohammad al-Zawi, the secretary-general of the Libyan parliament, said his country was facing a "barbaric" attack, and reiterated that Libyan forces had been observing a "ceasefire".