Several countries, including Israel, which scattered Lebanon with cluster bombs earlier this year, have not signed up.
Under the treaty, 25 signatory countries must start removing unexploded shells, grenades, rockets and cluster bombs left over from conflicts or pay for their removal, under the Protocol on Explosive Remnants of War, signed in 2003.
The International Committee of the Red Cross said: "This is the first international agreement to require the parties to an armed conflict to clear all unexploded munitions that threaten civilians, peacekeepers and humanitarian workers once the fighting is over."
The world's main military powers have taken part in formulating the document, but key countries such as China, Russia, Israel and the US have not signed.
Israel under scrutiny
Israel has come under pressure after using cluster bombs in its month-long bombardment of southern Lebanon in July and August when it aimed at crushing Hezbollah Shia fighters in the country.
Since the conflict, the clearing of unexploded bombs has progressed slowly because Israel has refused to give details of the areas targeted by these devices.
Unexploded cluster bombs in Lebanon have killed 23 people and wounded 136 since the official end of fighting.
The ICRC has said that 95 to 98 per cent of cluster munitions are neither reliable nor accurate, while 10 to 40 per cent of bomblets scattered by a mother bomb fail to explode.
The start date for the new protocol coincides with a UN conference to review another proposed treaty that aims to ban specific types of munitions. The conference will be held in Geneva until November 17.
Eighteen countries have backed a new convention to ban cluster bombs, but such a ban has been opposed by key powers such as Britain and the United States.
A US official told the conference that Washington held the view that the alternative to cluster bombs was to use an increased number of high explosive rounds, which have a more devastating effect.
The burden of unexploded bombs
The protocol enforced on Sunday is an addition to an existing deal, the Convention on Certain Conventional Weapons. This protocol bans or restricts the use of some weapons that cause "unnecessary or unjustifiable suffering to combatants" or that indiscriminately affect civilians.
This covers some types of fragmentation shells, some landmines or booby traps, and incendiary devices in civilian areas. But according to the UN and ICRC, this is not enough.
They have called for an outright ban on cluster bombs, which release several hundred smaller bomblets when they explode.
Unexploded ordinance is a "constant threat" to 200,000 refugees and displaced people in Lebanon as well as hundreds of thousands of people returning to their homes and for aid workers, the United Nations said this week.
"Cambodia, Laos and Vietnam are still suffering from the burden of unexploded cluster munitions some 30 years after the end of conflicts there," said a UN official.
"As long as there is no effective ban, these weapons will continue to disproportionately affect civilians, maiming and killing women, children and other vulnerable groups," said Jan Egeland, UN undersecretary general for humanitarian affairs.