Lebanese troops have crossed the strategically important Litani river, moving into some areas of southern Lebanon for the first time in decades.
Lebanese troops have not controlled the south since the 1960s
The troops were warmly welcomed in southern villages, many of which were badly damaged in more than a month of conflict between Hezbollah and Israel.
France is to send 200 extra troops to bolster the UN force in the south.
Israel says it has passed control of half of its positions there to the UN, which is set to enforce the ceasefire.
Israel, Hezbollah and the Lebanese government have all pledged to uphold a UN Security Council ceasefire resolution requiring the withdrawal from southern Lebanon of Israeli troops and Hezbollah fighters.
An expanded international force is to work alongside the Lebanese army in the south.
But despite France's initial commitment, Paris is still debating whether to send significant numbers of troops, and, along with other likely UN contributors, wants its role in the region clearly defined.
Lebanese officials have indicated they will not disarm Hezbollah, and the issue is likely to be high on the agenda at a UN meeting later on Thursday that will discuss the composition of the international force.
Israel's foreign minister Tzipi Livni said on Thursday that the speed of its full withdrawal from Lebanon would depend on how quickly UN peacekeepers were deployed to the region.
The Lebanese army headed south early on Thursday, dozens of vehicles crossing the Litani river using temporary bridges set up to bypass bridges damaged by Israeli shelling.
The 2,000-strong initial Lebanese deployment is due to rise to 15,000, as agreed by the country's cabinet - which includes two Hezbollah ministers.
People in the town of Marjayoun, scene of heavy fighting, waved Lebanese flags, threw rice and held an official ceremony to welcome the troops.
"Today is a new beginning for us in south Lebanon. We'll need some time to feel safe but it's a great start," George Najm, 23, told the Associated Press news agency.
Lebanese Brig Gen Charles Sheikhani hailed the deployment: "Since 1968 the army has not come here. This is our first time since then," he told AP.
Palestinian guerrillas, an Israeli occupation and Hezbollah fighters have by turns dominated southern Lebanon in the past four decades.
But there were few signs that the new reality in the south would lead to the disarmament of Hezbollah.
UN Security Council resolution 1701 does not compel the group to disarm, deferring the question to a second stage of negotiations.
Lebanese Prime Minister Fouad Siniora has said that the Lebanese army would have full authority the entire country, adding that there should be "no other apparent weapons except their weapons... no weapons outside the authority of the Lebanese state".
UN PEACEKEEPING FORCE
Likely contributors: Italy, Turkey, Malaysia, Indonesia
Other possible contributors: Morocco, Spain, Belgium, Finland, Brunei, Germany, Portugal, Pakistan
Current Unifil force: 2,000 troops from China, France, Ghana, India, Ireland, Italy, Poland and Ukraine
The country's information minister said he foresaw no "confrontation" with the group, while Emile Lahoud, Lebanon's president, has praised Hezbollah and said it should not be disarmed.
Italy's foreign minister also said Italian troops, preparing to join the international force, were not expecting to be involved in disarming the group.
Israel government spokesman Mark Regev said the UN resolution called for the creation of a "Hezbollah-free zone" south of the Litani.
But the BBC's Jon Leyne says there seems to be a tacit agreement for the fighters to hide their weapons and go underground.
As diplomatic manoeuvres continued at the UN and the military equation shifted on the ground, other parts of Lebanon began to emerge from the conflict.
The first commercial flights in more than a month landed at Beirut's international, which was disabled early in the conflict.
Two passenger flights from the Jordanian capital, Amman, landed at the airport, as well as a British aid flight.
In the south, Israeli aircraft have been dropping leaflets warning refugees to stay away.
Despite these warnings, there has been a steady stream of displaced people heading home.
Officials from the World Food Programme said trucks carrying aid packages were heading into the worst-hit regions, such as Tyre, Sidon and the town of Bint Jbeil, near the Israeli border.
Thousands of Lebanese returning home are finding their homes destroyed by bombing, as well as the threat of unexploded bombs.