Colombia's President Juan Manuel Santos has welcomed former Farc rebels as they took their seats at the opening session of parliament.
He said he was happy to see ex-rebels who fought the state for over five decades now respecting the constitution.
The new members of congress took up their seats on Friday as part of the peace deal reached in 2016.
The president urged his successor, Ivan Duque, to defend peace in Colombia.
"Today we are witnessing a true milestone in our history," said Mr Santos, who ends his term in office next month.
The Farc, or Revolutionary Armed Forces of Colombia, is now a political party known as the Revolutionary Alternative Common Force.
It has five seats in the Senate and five in the House of Representatives.
But rebel commander Ivan Marquez declined to take up his designated seat in protest at the detention of a fellow former rebel leader, Jesus Santrich, who also has a congressional seat reserved for him.
He is wanted by the US on drug trafficking charges.
President-elect Duque wants to prevent former rebels from sitting in congress until they are investigated and pay for crimes they may have committed during the civil war.
The former president, Alvaro Uribe, who heads Mr Duque's Democratic Centre Party, condemned their presence in parliament.
In a tweet, he described a "Congress with people convicted of atrocious crimes, without paying reparations, without fulfilling symbolic sanctions".
Analysts say the former rebels will not be welcomed by many in Congress, but believe that President-elect Duque will find it very difficult to make substantial changes to the peace agreement.
The 2016 deal was ratified by Congress and Mr Santos was awarded the Nobel Peace Prize for bringing it about.