Jesus Santrich, 52, hailed it as "a new step in the fight and defense of peace for Colombia," in comments to reporters after taking the oath of office.

He assumed his position in Colombia's lower house as one of 10 former guerrillas guaranteed a seat in Congress -- one of the terms of the historic 2016 peace accord that ended a half century of armed conflict by the Revolutionary Armed Forces of Columbia (FARC.)

That deal turned FARC into a political party, the Common Alternative Revolutionary Force, which uses the same FARC acronym as its predecessor rebel fighters.

However, Santrich, whose real name is Seuxis Paucias Hernandez, was prevented from taking his seat last year after being detained in April 2018 on suspicion of participating in the trafficking of 10 tons of cocaine to the US, which is seeking his extradition.

His FARC party released a video of Santrich, who is blind, swearing an oath before the lower house's vice president Atilano Giraldo.

The House of Representatives' press office confirmed the ceremony had taken place at the Congress.

Santrich was released from custody less than two weeks ago following an order by the Supreme Court for his immediate liberation.

A previous order by the special peace court tasked with judging crimes committed during Colombia's long armed conflict saw Santrich released last month, only to be re-arrested by authorities as he left prison.

The peace deal agreed by former Colombia president Juan Manuel Santos with the FARC stipulated that former guerrillas who commit crimes after the pact was signed be tried in a normal court and lose the benefits afforded by the accord, such as a ban on extradition.

The accusations against Santrich, who professes his innocence, crucially date from June 2017 to April 2018, after the peace accord was signed.

Under pressure from Washington, current President Ivan Duque's government was firmly opposed to releasing Santrich and has reaffirmed its intention to extradite him to the US.

Duque on Tuesday called Santrich "a mafioso" who is making a mockery of Colombian society.

The FARC political party said the president's remarks "border on slander."