Disarming fighters will get counselling and training
Hundreds of ex-Liberian Government fighters have staged angry protests to demand immediate payment for disarming.
They fired their guns into the air and beat passers-by near a United Nations-run disarmament camp.
The UN started taking weapons from some 40,000 members of government and rebel militias, many of whom are child soldiers, high on drugs, on Sunday.
Some 4,500 UN peacekeepers have deployed to Liberia in a bid to end 14 years of brutal civil war.
The fighters will be paid $300 but only half has been paid upfront.
"This gun is my life, if I don't get money I won't hand in any weapons," said Lieutenant Paul Dust, a rocket-propelled grenade in his hands and a pistol tucked in his trousers.
Under the UN scheme, which is due to last nine-months, the fighters will also be given counselling and vocational training.
Launching the disarmament programme, UN envoy Jacques Klein said: "This is Liberia's last chance. Liberia must put an end to war, or war will put an end to Liberia."
Some of the soldiers who handed in their weapons were children, including 13-year-old Richard Jack, once an "officer" with President Taylor's feared Anti-Terrorist Unit (ATU).
Fourteen years of was has devastated Liberia
Holding an AK-47, he said: "I started fighting at the age of six. I decided to turn in my gun because I want to go back to school."
The disarmament deal was drawn up after President Charles Taylor's departure into exile in August paved the way for a peace deal between government forces and the main rebel factions.
That followed a 14-year civil war which killed more than 200,000 Liberians.
Disarming and reintegrating the fighters into society is regarded as crucial to draw a line under the bloody conflict.
Rebel fighters are due to assemble in camps before the end of the year to hand in their weapons.
A total of 15,000 UN troops are expected to form a peacekeeping force in Liberia to monitor the disarmament, although there are only 4,500 in the country at the moment.