Colombia's President Juan Manuel Santos has met 30 rebels of the ELN, the country's second largest armed group, who laid down their arms.
Mr Santos called the development "a great step towards peace".
The ELN, or National Liberation Army, is not part of the peace talks with Colombia's largest rebel group, the Farc, but its leaders have expressed interest in joining the negotiations.
The government has insisted that the ELN must first release its hostages.
The left-wing group has around 1,500 members, officials say.
The 30-strong group surrendered their arms in the south-eastern region of Cauca.
Speaking in Cali, Mr Santos personally greeted the rebels, among them three pregnant women, who surrendered their arms and equipment.
"This is what the [peace] process is about. So every member of the ELN and the Farc follows their path fighting for their ideals, but without violence and without arms," he said.
This is the biggest single ELN contingent to surrender, Mr Santos said.
On Monday, Farc's chief peace negotiator said the armed conflict that has lasted nearly five decades was nearing its end.
Ivan Marquez, who is taking part in talks with the Colombian government in Cuba, called on left-wing parties and unions to join the effort to achieve peace.
The government wants to sign a peace accord by November.
But Mr Marquez warned against rushing into a settlement.
"It is possible [to reach an agreement by November]. But to achieve peace you need time. A bad peace deal is worse than war," he said in an interview with Colombian network RCN.
The first direct talks between Colombia's largest rebel group and the government were launched in November last year.
So far, however, the ELN has been left out of the talks.
Last month, in what was seen as an attempt to get the rival rebels to join the talks, the Farc issued a statement saying the two groups were discussing "unification".
But the government insists that the rebels must first lay down their arms and surrender all hostages.
The group recently released a Colombian soldier but is believed to be holding other hostages.
Chief government negotiator Humberto de la Calle said the aim of the Cuban peace talks was to get the rebels to give up their armed struggle and join the political process.
Agreement has already been achieved on land reform, but the negotiations are continuing on five other items on the agenda.
The Farc negotiator said recently that a Constitutional Assembly should be called to endorse the agreements reached in Cuba.
The Revolutionary Armed Forces of Colombia (Farc) are thought to have some 8,000 fighters, down from about 16,000 in 2001.
This is the fourth attempt at a negotiated peace deal since the beginning of the conflict in the 1960s.