Opening international talks in London, the Prime Minister called for the creation of a "contact group" to provide sustained political support for the country as it undergoes change.
"We are all here united in one purpose and that is to help the Libyan people in their hour of need," he said
"Today I believe should be about a new beginning for Libya - a future in which the people of Libya can determine their own destiny, free from violence and oppression.
"But the reason for being here today is that the Libyan people cannot reach that future on their own."
Mr Cameron said that the international coalition would continue the action needed to implement United Nations Security Council resolutions "as long as is necessary" to protect the population from attacks by Colonel Muammar Gaddafi's forces.
"As I speak, the people of Misrata are continuing to suffer murderous attacks from the regime. I have had reports this morning that the city is under attack from both land and sea," he said.
"Gaddafi is using snipers to shoot them down and let them bleed to death in the street. He has cut off food, water and electricity to starve them into submission. And he is harassing humanitarian ships trying to get into the port to do what they can to relieve their suffering.
"He continues to be in flagrant breach of the UN Security Council resolution."
At the same time, Mr Cameron said that the international community needed to ensure the delivery of humanitarian aid - including to newly "liberated" towns - while helping plan for the future after the current conflict is over.
"When the fighting is over, we will need to put right the damage that Gaddafi has inflicted - repairing the hospitals ruined by shells, rebuilding the homes demolished by Gaddafi's tank rounds, and restoring the mosques and minarets smashed by his barbarity," he said.
"It's never too early to start planning co-ordinated action to support peace in Libya over the long term."
Mr Cameron called for continued international pressure on Gaddafi's regime.
"Our military actions can protect the people from attack, and our humanitarian actions can help the people recover. But neither are sufficient to provide the path to greater freedom," he said.
"Ultimately, the solution must be a political one - and it must be for the Libyan people themselves to determine their own destiny.
"That means reinforcing the UN sanctions to exert the greatest possible pressure on the Gaddafi regime."
The conference brings together all members of the coalition involved in the military operation, as well as the United Nations, Nato, the African Union and the Arab League.
Although they are not part of the formal proceedings, representatives of the main opposition group are in London for separate talks with key participants.
Mr Cameron is due to welcome Mahmoud Jabril, special envoy of Libya's Interim Transitional National Council, to Downing Street for discussions this afternoon.
The high-level talks came as rebel fighters were forced back from dictator Muammar Gaddafi's home town of Sirte as their rapid westward push was halted.
Despite more than a week of coalition air strikes against regime military targets, they were met by considerable firepower in what could become a decisive confrontation.
US Secretary of State Hillary Clinton said the international community had to "speak with one voice in support of a transition" to a political future chosen by the Libyan people.
It would require a "broadly inclusive process that reflects the will and protects the rights of the Libyan people", she said including the interim council - with whom she met earlier.
And it required a "unified front of political and diplomatic pressure that makes clear to Gaddafi he must go, sends a strong message of accountability and sharpens the choice for those around him".
The military effort had reached "a turning point" with full command of the operation set to pass from the US to Nato tomorrow.
America is scaling back its involvement to a more supporting role, including intelligence, search and rescue and communications jamming capabilities.
Mrs Clinton praised the UK Government's "critical leadership effort" as well as the role played by France in securing the co-ordinated international action.
Arab countries Qatar, Iraq, Jordan, Morocco, Lebanon, Tunisia and the United Arab Emirates are attending amid efforts to "strengthen and broaden" the alliance behind military action.
Sheikh Hamad bin Jassim bin Jabor Al-Thani, the prime minister of Qatar which has recognised the interim council and contributed fighter jets to the coalition action, was among opening speakers.
He said the military operations had helped encourage ordinary Libyans to believe they were "about to get rid of that murderous regime and be able to achieve a better future and have freedom".
UN Secretary General Ban Ki-moon said he was sending his special envoy back to Libya after today's events in a bid to secure, in talks with both sides, a peaceful settlement.
Welcoming the plan for a contact group, Mr Ban said he would like to co-ordinate efforts.
"I know that everybody is very much anxious and passionate to help the Libyan people and to address this crisis," he said. "But we need to have very close co-ordination and I am going to lead this co-ordination, if you agree."
Years of dictatorial rule meant the Libyan people would need significant help in achieving a smooth transition to democracy.
Mr Ban said he was receiving "deeply-disturbing reports about the lack of protection of civilians, including migrant workers, as well as abuses of human rights by the parties to the conflict" - in breach of the Security Council resolution.
More than 380,000 had fled Libya since the start of the unrest and 13,000 remained stranded at the borders with Egypt and Tunisia, he said.
Food supplies were dwindling and a drawn-out conflict would worsen that situation.
"We are mobilising all possible efforts," to ease the humanitarian crisis, ne said, appealing to countries present to contribute "generously" to a UN flash appeal for emergency funds.
So far 67% of the 160 million dollars (£102 million) sought had been raised, he told them.
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