* Solicitor general files legal challenge to peace process
* Constitutional court expected to rule in July
* Critics say legal framework for peace is amnesty in disguise
By Helen Murphy and Eduardo Garcia
BOGOTA, April 5 (Reuters) - Colombia's top prosecutor on Friday filed a legal challenge to a law that paved the way for ongoing peace talks, a move that could undermine government efforts to end five decades of war.
President Juan Manuel Santos drew up a legal framework last year that led to peace negotiations with Latin America's biggest rebel group, the Revolutionary Armed Forces of Colombia, or FARC. The bloody conflict has killed tens of thousands and caused damage to the Andean nation's economy.
Solicitor General Alejandro Ordonez on Friday asked the constitutional court to certify that parts of the so-called Legal Framework for Peace are illegitimate because they could leave serious crimes unpunished.
Ordonez, one of Colombia's most reputed jurists, objected to articles that would allow FARC commanders to run for election and hold public office, providing they have not committed crimes against humanity.
The framework has been condemned by some human rights advocates, as well as influential opposition leaders like former President Alvaro Uribe, a fierce critic of the peace process.
They claim the law is an amnesty in disguise that would allow guerrillas responsible for atrocious war crimes to benefit from soft prison sentences or walk away scot-free.
"The framework for peace makes it impossible to guarantee the rights to find the truth, justice, and reparation to all victims of Colombia's armed conflict," Ordonez told reporters.
He said that under the law some war crimes, including human rights abuses and genocide, will not be investigated, which would allow rebel leaders to escape prosecution.
"Without guaranteeing rights for the victims, it's impossible to guarantee it won't happen again," he said.
Ordonez, a close ally of Uribe, the provision will violate certain international treaties because it would effectively pardon crimes against humanity.
The court was expected to rule by July, a source told Reuters.
The Colombian government has been holding peace negotiations with guerrilla leaders in Cuba since November.
FARC leader Pablo Catatumbo will join rebel negotiators already in Havana for peace talks, a security source said on Friday, in a bid to rally more support from rank and file guerrillas to end Colombia's five-decade conflict.
Catatumbo, who is sought by the United States for drug trafficking, has already left Colombia for Cuba, said the source, who asked not to be identified.
Government negotiators and FARC leaders have been in talks since November to find a way to end the conflict that has killed tens of thousands since it began in 1964.
Some observers reckon the current FARC team, headed by Ivan Marquez, does not represent the entire group. Catatumbo is one of the FARC's six-member leadership known as the Secretariat.
The group is at its weakest in decades after a U.S.-sponsored army drive that killed the guerrilla's top commanders and forced its units into remote areas. But with around 8,000 troops, the FARC is still strong and has stepped up attacks against oil and mining installations in recent months.
The two sides are discussing rural development and land reform, the first issues on a five-point agenda, with the aim of addressing the primary cause of the conflict - the South American country's long history of social inequality and land ownership concentrated in the hands of a few.
The government and the FARC in March said they had made enough progress that they asked the U.N. office in Colombia to start preparation for a public forum on their next agenda item, the FARC's future political participation.
The FARC first took up arms in the 1960s as a Marxist group struggling against inequality, but later turned to kidnapping and drug-trafficking to finance itself.
The group is popular in rural areas where hospitals and schools are scarce and many people feel they are not benefiting from the economic boom that city-dwellers are enjoying. (Reporting by Helen Murphy)