The UN-brokered treaty is the first covering weaponry of any kind for more than a decade and aims to bring transparency and protection of human rights into the often dubious USD 85 billion a year global conventional arms trade.
The treaty covers tanks, armoured combat vehicles, large-caliber artillery, combat aircraft, attack helicopters, warships, missiles and missile launchers, as well as the vast trade in small arms.
The start of signing was described as an "extremely important milestone" by ministers and other representatives of Argentina, Australia, Britain, Costa Rice, Finland, Japan and Kenya, which sponsored the first 2006 UN resolution calling for treaty talks.
"It is vital that the treaty comes into force as soon as possible and is effectively implemented," the seven said in a statement before Argentina became the first of 61 countries to sign the treaty on the first day.
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"It is only then that the international community will have an effective tool to address the unregulated and illicit conventional arms trade that causes unspeakable human suffering," the statement by the seven added.
The UN General Assembly passed the treaty in April when 154 countries voted in favor, but Syria, North Korea and Iran voted against and Russia, China, Egypt and India were among 23 countries to abstain.
Russia and China are not expected join the treaty any time soon.
Germany's Foreign Minister Guido Westerwelle called the treaty a "historical breakthrough" but said more should be done to control the arms trade.
Britain's deputy foreign minister Alistair Burt said doubters of the treaty were now under pressure to join.
"The force behind so many states wanting to conclude an arms trade treaty after so long meant something. The world is now different," he told reporters.
The United States, the world's biggest exporter of arms and ammunition, backed the treaty but was not among the first signatories.