The FARC, or Revolutionary Armed Forces of Colombia, has been fighting the government in a jungle and urban conflict that has killed more than 200,000 people in the five decades since it began as a peasant movement seeking land reform.
The partial agreement may pave the way for FARC to enter Colombian politics, which lead government negotiator Humberto de la Calle said would provide a "new democratic opening" that will lead to peace after an end of conflict. "Never again politics and weapons together."
Like other Latin American guerrilla groups, the FARC aspires to become a political party if a peace deal is signed.
"We are completely satisfied with what we have agreed on the point of political participation," FARC leader Ivan Marquez told Reuters. "We are doing well; in no other peace process have we advanced as much as we have here in Havana. We have taken an important step in the right direction to end the conflict and to achieve a real democracy in Colombia."
President Juan Manuel Santos, facing a barrage of criticism from the opposition for negotiating with the rebels, wants to show progress in the talks that have dragged on for a year and, until now, have yielded only incomplete agreement on the first of a five-point agenda.
The center-right Santos has seen his approval ratings slump in the last few months, partly due to the perception that he has offered too many concessions to the rebels in return for little.
Partial accord has been reached on land reform from an agenda that also includes reparation to the FARC's victims, tackling Colombia's drug trade, and an end to fighting.
Wednesday's development will likely lift Santos' popularity and provide momentum should he decide by the November 25 deadline to seek a second presidential term. Many believe a second term hinges on progress at the negotiating table.
The two sides are unlikely to reveal many details of the agreement, but Colombians will be looking for clues on how much government negotiators have offered the rebels and how they will pay for their crimes. Many will be unwilling to accept FARC leaders being given seats in congress - as the rebels have demanded - without first receiving jail terms and then passing through the electoral process.
While most Colombians are desperate to see an end to the war, initial euphoria over negotiations has worn off as many doubt talks will soon reach a successful end.
"Even though the agreement is partial, what was announced today is a defeat to those who have sought to darken or end the peace process and constitutes a strong basis to believe we could reach a definitive peace," Ivan Cepeda, a leftist congressman, said.
Opposition leaders like former President Alvaro Uribe are furious that the FARC has tried to dictate government policy while it continues to bomb economic infrastructure and kill civilians and military personnel.
(Reporting by Helen Murphy and Luis Jaime Acosta in Bogota, Marc Frank, Nelson Acosta and Rosa Tania Valdes in Havana. Additional reporting by Peter Murphy; Editing by Vicki Allen and Jackie Frank)