* Government, guerrillas to start talks in Cuba next week
* FARC wants constitutional assembly in future
* Rebels reiterate call for citizen participation
* Government rejects request to widen agenda
By Jack Kimball
BOGOTA, Nov 15 (Reuters) - The top FARC rebel negotiator wants a national assembly to change the constitution once a peace deal is reached with the government, according to an interview on a left-wing website published on Thursday.
Talks to bring an end to Latin America's longest-running insurgency will begin in Cuba on Monday when Colombia's government and the Revolutionary Armed Forces of Colombia, better known by its Spanish acronym FARC, sit down for the first round of a five-point peace agenda.
Negotiations were supposed to start this week but were delayed so negotiators could work on "technical" details, including the participation of civil society and other groups.
Ivan Marquez, head of the FARC negotiating team, said that citizens should have a voice at the table, which would give any agreement more legitimacy, and that a peace deal would also need to have "legal and constitutional" force.
"We think that a good way to achieve that is to provide the possibility, later, for a national constitutional assembly," Marquez said in an video interview published on the Anncol website, which routinely carries FARC statements.
In the interview recorded in Havana, Marquez, a member of the FARC's seven-member ruling secretariat, appeared seated in front of a stone wall. The video was uploaded on Thursday but apparently recorded late last week.
President Juan Manuel Santos' government has flatly ruled out discussing major changes to Colombia's economic or political model, saying that if the guerrillas want to modify the system, they should join the political process and win elections.
More than 20 years ago, Colombia held a nationwide assembly to rewrite the 1886 constitution, in which demobilized guerrillas from smaller rebel groups participated, but not the FARC or the National Liberation Army, another left-wing group.
The Marxist FARC guerrillas gained fighters and territory after the 1991 constitution until the 2000s, when a U.S.-backed military offensive began to thin their ranks and push them into remote hideouts.
Santos, first as defense minister under former President Alvaro Uribe and later as Colombia's leader, has dealt the group some of its biggest blows, killing senior leaders and hitting units responsible for financing operations.
Negotiations were formally launched on Oct. 18, but the talks immediately got off to a rocky start after the guerrillas said they wanted to discuss a range of topics not specifically mentioned in the agenda.
The government has ruled that out, saying issues not listed in the agenda, especially rebel demands for foreign mining companies to stop operations, would not be negotiated.
On a trip to Spain on Thursday, Santos reiterated that there would be no ceasefire until a final deal is hashed out.
"These negotiations cannot be overly long. One of the conditions that we set at the outset is that we will not lower our guard on the military front," Santos said.
"There are some who are proposing a truce, a ceasefire. The answer has been clear and forceful. No truce or ceasefire ... the sooner we reach a deal the sooner the guns go silent."