One attack by one of several active Left-wing movements killed 15 soldiers in the province of Norte de Santander, close to the border with Venezuela on Monday.
Three other soldiers were also killed in two separate attacks by the Revolutionary Armed Forces of Colombia, or FARC.
The deaths mark one of the deadliest days for the military since President Alvaro Uribe took power in 2002.
The main attack occurred when 18 soldiers traveled to investigate reports of an illegal checkpoint on a highway, the commander of the armed forces, Gen. Carlos Ospina, told a news conference.
At some point they were attacked by guerilla forces who killed 15 soldiers, leaving only three survivors.
"It's deplorable, but the soldiers who were trying to establish security in the region walked into a mine field," Ospina said, speaking figuratively.
Rebels from all of Colombia's main illegal groups operate in the region, about 460 kilometers (285 miles) northeast of Bogota.
Also on Monday a car-bomb exploded in Bogota as trucks carrying dozens of soldiers passed by.
The blast killed one civilian and injured ten soldiers. Car-bombs are now a rarity in the Colombian capital as the security forces have increased the number of roadside inspections of vehicles.
In Narino, in southwestern Colombia, another explosion killed two soldiers and injured six more.
Alvaro Uribe, Colombia's tough-talking president
Bogota is on maximum alert ahead of Uribe's inauguration on August 7.
Colombia, the world's biggest cocaine exporter, often sees an increase in violence at election and inauguration time.
Four years ago Uribe's first swearing-in was overshadowed by FARC missile attacks that killed 21 people and injured 60.
FARC has an estimated 17,000 armed members. It was formed more than 40 years ago and says it is fighting for equality in a country with huge gaps between rich and poor.
Uribe, whose father was killed by rebels in a botched kidnapping more than 20 years ago, is popular for his U.S.-backed crackdown on the guerrillas.
He won re-election in a 62-percent landslide in May.
Uribe is also credited with reducing crime, particularly in urban areas, and opening roads long off limits due to guerrillas who fund themselves partly through taxing cocaine smugglers and kidnapping.