Twenty-five Colombian troops have been killed in two devastating attacks by Marxist rebels.
The Farc is one of the world's most powerful guerrilla armies
Farc rebels attacked troops in Putumayo province near the Ecuador border, and fought for control of a road near the Venezuelan frontier.
The attacks left the worst death toll in a single day since President Alvaro Uribe came to power three years ago.
The BBC's Jeremy McDermott in Bogota says the rebels want to undermine Mr Uribe and ensure he is not re-elected.
The killings are part of a counter-offensive begun in February that has killed more than 120 members of the security forces.
In one incident, 19 soldiers were killed after up to 300 guerrillas ambushed an army convoy during an attack apparently targeting oil wells near Puerto Asis in Putumayo province.
Aircraft were sent to the scene and attacked rebel positions, army officials said, adding that the Farc had also suffered casualties.
Nineteen soldiers who went missing during the assault were later found safe and well. A spokesman said they had become separated from their units during the attack.
In more fighting near the Venezuelan border, 400km north-east of the capital Bogota, six troops were killed as they fought with rebels to gain control of a road linking Ocana and Sardinata.
At the start of this year, the army said they had the rebels on the run and that the security policy of hardline president Alvaro Uribe had forced the guerrillas into irreversible decline.
But our correspondent says the wave of attacks across the country since February has proven the military wrong.
Buoyed up by hundreds of millions of dollars earned from drugs, the Farc (Revolutionary Armed Forces of Colombia) seem to have taken the initiative away from the security forces, he says.
The guerrillas hope to destroy President Uribe's re-election hopes by showing that his security policy, a cornerstone of his administration, has failed.
The guerrillas want the next president to be more disposed to talk peace - but on their terms.