Nations at the conference also called on major arms producers such as China, Russia and the United States to join them in signing the treaty.
Dropped from warplanes or fired from artillery guns, cluster bombs explode in mid-air to randomly scatter hundreds of bomblets, which can be just 8cm large.
Many bomblets fail to explode, littering war zones with de facto landmines that can kill and maim long after a conflict ends.
Handicap International (HI), a campaign group, says about 100,000 people have been killed or maimed by cluster bombs worldwide since 1965, with 98 per cent of them civilians.
More than a quarter of the victims are children who often mistake the bomblets for toys or tin cans.
Richard Moyes of the Cluster Munitions Coalition (CMC), an umbrella group that comprises some 300 non-governmental organisations, said: "The world is a safer place today.This is the biggest humanitarian treaty of the last decade."
Laos, the country most affected by cluster bombs, was the second nation to sign Wednesday's treaty at Oslo city hall.
Between 1964 and 1973, the US air force dropped 260m cluster bombs on Laos, or the equivalent of a fully-loaded B-52 bomber's payload dropped every eight minutes for nine years.
|HI says about 100,000 people have been killed or maimed by cluster bombs since 1965 [AFP]
Over two days, dozens of countries, including Britain, Canada, France and Germany, are to sign the treaty, which was finalised in Dublin in May.
The final number of signatory states will only be known at the end of the ceremony on Thursday.
"We hope to see more states signing in the coming weeks, the coming months, the coming years," Stoere said,
However, the world's biggest producers and users of cluster bombs have refused to sign the ban.
Stoere said: "Of course, [the treaty] would have been a stronger instrument if we had the US, Russia, China, Israel, Pakistan and India onboard.
"But we're creating a new norm and we're going to take away a huge market for the producers."
Washington reiterated its opposition to the ban on Tuesday.
"Although we share the humanitarian concerns of states signing the CCM, we will not be joining them," the state department said in a statement when asked for its views on the signing ceremony.
"The CCM constitutes a ban on most types of cluster munitions; such a general ban on cluster munitions will put the lives of our military men and women, and those of our coalition partners, at risk."
Despite the absence of key countries, opponents of cluster bombs say the treaty, also known as the Oslo Convention, should help stigmatise the use of such weapons even by non-signatory countries.
Jens Stoltenberg, the Norwegian prime minister, said: "The treaty places moral obligations on all states not to use cluster munitions."
Moyes said: "The treaty will increase the political cost of using these weapons for any country, even countries that don't sign will struggle to use these weapons in the future."
The CMC group is hopeful that the US position will shift when Barack Obama, the US president-elect, moves into the White House on January 20.
In 2006, Obama voted in the US Senate to ban the use of cluster munitions in heavily populated areas, but in the end the motion was rejected.