* "Concrete advances" cited but major differences remain
* Negotiations to break for holidays, resume Jan. 14
By Jeff Franks and Nelson Acosta
HAVANA, Dec 21 (Reuters) - Talks between the Colombian government and Marxist-led FARC rebels to end their bloody, half-century-long conflict have made progress, but broad areas of disgreement lie ahead, the two sides said on Friday as they headed into a holiday break.
Lead government negotiator Humberto de la Calle, speaking at the conclusion of the second round of meetings, said the two sides had agreed on some "unprecedented mechanisms" for civic input into the peace process.
"Since November 19 when we formally began, we've had 21 sessions and more than 100 hours of intense work, and concrete advances, all as expected," said de la Calle, who read a statement but did not take questions.
The two sides are trying to end a war that dates to 1964 when the FARC, or Revolutionary Armed Forces of Colombia, was formed as a communist agrarian movement to end the country's long history of social inequality.
Tens of thousands of people have died and millions have been displaced in South America's last Marxist-led armed rebellion, a vestige of the Cold War.
They said in a joint communique on Friday the talks, which will resume in Havana on Jan. 14, were being held "in an atmosphere of respect and constructive spirit."
But the comments of de la Calle and, in a separate press conference, the rebels, signaled the differences to be overcome.
De La Calle said the goal was to convert the illegal guerrilla group into a legitimate political organization, while maintaining the country's economic and social models.
"They don't have to abandon their ideology and the government does not have to change its model of society," said the former vice president.
"We are not negotiating the development model of Colombia nor the democratic system of the government."
CHANGES NEEDED 'WITHOUT A DOUBT'
But lead FARC negotiator Ivan Marquez, speaking later, said achieving peace "without a doubt requires changes to the model of society and the anti-democratic political system that in the end is responsible for shameful inequality and exclusion that characterize the Colombian regime."
"A badly constructed peace is worse than a war," said the bearded, bespectacled Marquez.
Three previous peace attempts have failed, but Colombian President Juan Manuel Santos hopes a decade-long, U.S.-backed offensive has weakened the rebels enough that they will want to end the fighting on the best possible terms.
If peace is not achieved, the FARC still has 9,000 troops that can keep inflicting damage on the continent's fourth-largest economy.
The negotiations, which are being held in a Havana convention center in the ritziest part of the Cuban capital, have begun with the topic of rural development, the first of six points to be addressed.
Others range from the rebels' involvement in the illicit drug trade to their future political participation.
De la Calle said Colombians, whose opinions are being sought online and in public forums, had provided nearly 3,000 suggestions to aid the peace process, some of which were being closely studied.
He said negotiators were also receiving ideas from development experts and peasants about how to improve life for the rural poor in Colombia.
The FARC's Tanja Nijmeijer, a Dutch national, said the rebels had received more than 250,000 suggestions from Colombians on its websites.
When the talks began in November, the FARC declared a unilateral ceasefire until Jan. 20 and complained this week that the government had not joined it in laying down arms.
Instead, the government has kept up its offensive, killing at least 20 FARC guerrillas in one attack earlier this month.
"The continuation of the fighting severely affects the progress of the Havana process," said rebel negotiator Andres Paris. He said the FARC wanted to give Colombians a peaceful Christmas, but warned that "this unilateral decision ends Jan. 20."
Santos has vowed to maintain military pressure instead of allowing the rebels to regroup as they did during previous failed peace talks a decade ago.
He wants to peace process wrapped up by November 2013, although the rebels said it could take much longer.