Avella and Sentor Carlos Lozano, who were travelling together, were not hurt, Arauca police chief Col. Camilo Ernesto Alvarez Ochoa said.
"Dr. Avella was on the road between Fortul and Puerto Jordan when a motorcycle carrying two individuals pulled up to them, signalled them to stop and fired on the vehicles carrying the bodyguards," Alvarez Ochoa told reporters.
Gunfire was exchanged, but the vehicles continued on the road until they reached an army checkpoint, the police chief said.
Avella and Lozano travelled to Tame on Friday to meet with Patriotic Union supporters over the weekend in Arauca, which is on the border with Venezuela.
The presidential candidate, who returned to Colombia in December after living abroad for 17 years, called on President Juan Manuel Santos on Sunday to guarantee the safety of her campaign.
"In the face of the attack in Arauca, we demand the right to live and to be an option for power! There will be no peace without guarantees!" Avella said in a Twitter post addressed to Santos and Defence Minister Juan Carlos Pinzon.
Officials have not determined who staged the attack on the presidential candidate, who is at the base in Puerto Jordan.