BOGOTA, Colombia -- Guerrillas attacked an army base, ambushed police and launched other attacks across Colombia on Saturday, killing 13 people, as the nation voted in a referendum seen as a test of President Alvaro Uribe's support.

Rebels of the Revolutionary Armed Forces of Colombia attacked a base of the army's Sixth Brigade in the western city of Ibague with explosives and gunfire early Saturday, killing one soldier and wounding two.

"We must face down this terrorist threat that has no limits," Uribe said. "We cannot let this day of democracy, aimed at bringing fundamental changes, go to waste."

FARC rebels also ambushed police patrols near the southwestern villages of Jambalo and Silvia, killing six officers.

A bomb planted by suspected rebels exploded outside a milk processing factory in the northwestern town of Yarumal, killing six people and wounding five. The manager of the Colanta factory is Jenaro Perez, a friend and political ally of Colombia's president.

FARC rebels also blocked a major highway leading from the third-largest city, Cali, to Buenaventura on the Pacific coast, and burned a half-dozen trucks.

A Colombian senator narrowly escaped that attack. Upon seeing the rebel roadblock, Sen. Juan Carlos Martinez's driver pulled a U-turn. The car came under a hail of rebel gunfire but no one aboard was hurt.

The attacks were the rebels' response to government attempts to not only strengthen its hand through the referendum on Saturday but to also stage municipal and state elections through the country today.

The U.S. Embassy in Bogota warned Americans that terrorist attacks "could specifically target popular commercial and nightlife centers, including shopping complexes, restaurants and populated areas with concentrations of such establishments."

Gen. Jorge Enrique Mora, the commander of Colombia's armed forces, discounted the effects of the rebel attacks.

"These threats will not interfere with Colombia's democratic fiesta," Mora told RCN radio.

Even before the election-day violence, there were indications that voter apathy could undermine the referendum. To be valid, at least 25 percent of registered voters, or 6.2 million people, needed to participate.

A recent Gallup poll found that only 23 percent of voters would do so.

Uribe urged voters to turn out as their patriotic duty.

"The referendum is not a miracle but it is a step against corruption and political misconduct," Uribe said before voting in Bogota's main Plaza Bolivar. "It is a step toward strengthening public order."

Colombia's popular president has campaigned tirelessly for the referendum, saying it is vital to defeat terrorism and prevent an Argentina-style economic collapse.

Its 15 proposals would give Uribe a freer hand to clamp down on corruption, ban tainted politicians from holding office, make congressional spending more accountable and cut the number of seats in Congress from 267 to 218.

It also includes plans to freeze public-sector salaries for at least two years and cap state pensions in an effort to trim spiraling deficits -- giving the government more money to spend on defense.

Moreover, a "yes" vote would be seen as a popular endorsement of Uribe's campaign to crush leftist guerrillas and destroy the illicit drug crops that supply them with cash to buy guns. A defeat, on the other hand, could weaken his political standing.

The measures are staunchly opposed by trade unions, leftist politicians and by some within the president's own party.