By JUAN FORERO

BOGOTÁ, Colombia, Jan. 4 — Colombian agents, working with American intelligence operatives, tracked a top Colombian guerrilla commander for months
before he was arrested Friday on a busy street in Quito, the capital of neighboring Ecuador, Colombian military authorities said Sunday.

With the capture of the commander, Ricardo Palmera, 53, Colombia's government registered its most important arrest of a rebel leader in the 40-year war against the
country's largest guerrilla group, the Revolutionary Armed Forces of Colombia.

Mr. Palmera, a leading negotiator in peace talks that failed in 2002 and a chief ideologue in the rebel group, was detained by the Ecuadorean police on Friday night in
what Ecuadorean authorities said was a routine document check. He was flown back to Colombia and is now being held by authorities here in Bogotá.

Military officials here said Colombian agents had been on his trail for six months as he crossed from Colombia into Ecuador, where Colombian news reports said he
was seeking medical treatment for prostate cancer. On Sunday, officials at Colombia's Defense Ministry showed reporters secret videos taken in recent weeks of Mr.
Palmera, well dressed and accompanied by a girlfriend and relatives, as he strolled through the Ecuadorean capital.

The government of Colombia is calling the capture a major blow against the Revolutionary Armed Forces of Colombia, the Marxist rebel group, and proof that
President Álvaro Uribe's efforts to weaken the rebels is working.

"This was not some coincidence, some simple operation against the undocumented," said a Defense Ministry official. "This was the work of military intelligence that
was going on for six months."

The capture has given Mr. Uribe a much needed lift after a rough few weeks that saw Colombian voters reject a referendum that would have given him more control
over state spending. His 17-month-old government had also been unable to capture any high-profile rebel commanders, though several midlevel leaders had been
killed or arrested.

"The government can say its policies are having results," said Alfredo Rangel, a military analyst in Bogotá and a former army adviser. "It gives the armed forces a great
sense of optimism and a spirit of triumph."

Colombia's security services have become more adept in their war against the rebels, thanks to hundreds of millions of dollars in American military aid, training and,
most recently, the sharing of information gleaned by American intelligence analysts. Although it was unclear what role the Americans played in Mr. Palmera's capture,
the minister of defense here, Jorge Uribe, called the American assistance "vital" in finding the guerrilla commander.

The United States monitors radio signals and phone conversations and uses aircraft for surveillance of rebel-held regions. American troops have also trained a
Colombian special forces unit whose sole purpose is capturing high-ranking rebel and paramilitary commanders. Many of those commanders are assumed to cross
porous borders into Ecuador and Venezuela, where they are hard to track.

Mr. Palmera, who is known here by his nom de guerre, Simón Trinidad, faces charges for crimes ranging from the murder of a former culture minister in 2001 to the
2002 rocket attack of a church that killed 120 people. Colombian authorities said the United States might also seek Mr. Palmera's extradition on drug trafficking
charges.

Mr. Palmera was well known to many journalists here as the de facto spokesman for the rebel group during ill-fated peace talks with Mr. Uribe's predecessor,
President Andrés Pastrana.

Well read and educated, he was different from other commanders of the rebel group, which draws most of its members from the Colombian peasantry. Mr. Palmera
comes from an aristocratic family and was a banker before joining the rebels in the 1980's because of what he said he viewed as society's injustices.