Alvaro Uribe, Colombia's current president, also spoke at the funeral.
Lopez, who was president from 1974-1978, died of a heart attack on Wednesday, aged 94.
Last month, 11 provincial legislators kidnapped by Farc were killed when the military launched an assault on a Farc camp.
Farc says the politicians were killed in the crossfire, but the government denies attacking the camp and says the rebels murdered the hostages.
Fabiola Perdomo, the wife of one of those killed in the raid, has also called for a "humanitarian agreement".
The 11 provincial politicians were among 56 hostages, including Ingrid Betancourt, a French-Colombian, and three Americans, who the rebels want to swap for 500 of its members held in prison.
After Farc announced the deaths, Lopez questioned Uribe's rejection of Farc's proposal to create a demilitarised zone to negotiate an exchange.
"It looks to me that a solution is not being sought, but rather a victory," he said.
Uribe released some 150 rebels from prison last month, hoping Farc would reciprocate by freeing hostages, but no exchange has taken place.