At a meeting of the Organization of American States (OAS), Colombian Ambassador Luis Alfonso Hoyos accused Venezuelan President Hugo Chavez's government of tolerating the rebels he said carried out killings, kidnappings and drug-trafficking on both sides of the long frontier.
"The continent cannot allow this nightmare to spread," Hoyos said.
Venezuela's ambassador rejected the Colombian presentation as a "lie."
"There is no evidence, no proof, these are photos taken I don't know where," Roy Chaderton said.
The accusations by Bogota have raised tensions again with Venezuela, where left-wing President Chavez has often been at odds with the conservative government of outgoing Colombian President Alvaro Uribe, the United States' top ally in the region.
The two Andean neighbors who share a long, porous border have squabbled on and off for years, stoking fears of an eventual military confrontation between the two oil producers.
Chavez's government has dismissed the latest Colombian accusations as "pathetic" and says they are aimed at derailing steps to repair bilateral ties before the inauguration of Juan Manuel Santos as Colombia's new president on Aug. 7.
Hoyos showed a series of photos and videos of alleged Revolutionary Armed Forces of Colombia, or FARC, leaders and guerrillas relaxing, roasting animals and playing a piano at camps he said were well inside Venezuela.
"Facts over recent weeks show that real risks are materializing due to the consolidated, active and growing presence of these terrorist groups in Venezuela," Hoyos said.
He added the Venezuelan authorities "tolerate the presence of these groups, they don't carry out actions against them." Sometimes the rebels were even accompanied by members of Venezuela's National Guard, Hoyos added.
FARC rebels carried out cross-border attacks from Venezuela as recently as June and July, Hoyos said as he showed photographs of civilian and military victims of these attacks.
He presented photos he said showed the presence of top FARC leaders, such as Ivan Marquez and Rodrigo Granda, at camps up to 23 km inside Venezuela. The encampments were named "Ernesto," "Berta" and "Bolivarian Camp," he said, citing information provided by rebel deserters.
These encampments served as "summer camps," where the rebels "can relax, put on weight and plan attacks," he said.
Presenting aerial photographs, Hoyos said Colombia was providing detailed coordinates of the sites, including one where guerrilla bomb-makers were trained.
He complained that Chavez's government had responded with "mockery and insults" to repeated complaints from Bogota about the presence of the rebels on Venezuelan soil.
He said this shelter allowed the FARC and other Colombian rebel bands to rearm and regroup at a time when Uribe's government had made major strides in halting violence, reducing annual kidnappings by 96 percent and homicides by 50 percent.
Hours before the OAS session, Colombia recalled its ambassador from Venezuela in what has been a tit-for-tat diplomatic dispute. Last week Venezuela's Chavez recalled his envoy from Bogota for consultations, and Colombia made the same move on Wednesday night.
Relations between socialist Chavez and Uribe's conservative government have deteriorated in the last two years because of spillover from Colombia's decades-long civil conflict and accusations that Chavez helps the leftist rebels.
Chavez said late on Wednesday he had instructed his ambassador at the OAS, Chaderton, to "uphold the battle" against the Colombian accusations. But he added: "We won't allow ourselves to be provoked."
The latest Colombian charges have further strained relations damaged by a 2009 deal that allowed US forces to use Colombian military bases for anti-drug operations.
Chavez, whose socialist rhetoric and policies have made him an anti-US standard-bearer, says the military deal threatens his OPEC nation and could herald a US invasion.
Colombia is the main US military ally in South America and has received billions of dollars in US aid to help fight the rebels, who are funded largely by the cocaine trade.
Chavez ordered a slowdown of trade between Venezuela and Colombia last year after the Colombian-US bases deal. Annual trade had totaled some $7 billion, and the cutoff exacerbated shortages and fueled high inflation in Venezuela, while slowing Colombia's economic recovery.
