Almost 7,000 members of Colombia's largest rebel group have arrived in special transition zones to begin disarming, completing a key step in a revised peace deal that seeks to end more than five decades of armed conflict.

Colombian voters appear to have rejected a peace deal with the country's largest rebel movement by a razor-thin margin in a national referendum, delivering a major shock to the war-torn nation.

Colombian president Juan Manuel Santos and the top Farc rebel commander have pledged to end the 50-year war within the next six months, sealing their pact with a handshake likely to stand as a lasting image in the South American nation.