The trip by Londono, who now heads the FARC's political party and is better known by his nom de guerre Timochenko, marks the first time he has attended an inauguration.
The 2016 peace deal between the government and Revolutionary Armed Forces of Colombia (FARC) put an end to the group's role in more than five decades of war in the Andean country that killed 260,000 people and displaced millions.
Londono's trip was approved by the Special Jurisdiction for Peace (JEP) tribunal, which is tasked with trying FARC and military leadership for human rights violations and war crimes.
Londono, 59, is allowed to remain outside of Colombia between Friday and Dec. 4, the tribunal said in a statement.
He traveled to Mexico City for the Saturday ceremony with another former guerrilla after being invited by Mexico's Labor Party, which is part of Lopez Obrador's coalition.
The FARC debuted as a political party in March legislative elections, receiving just 50,000 votes. It has 10 legislative seats guaranteed to them through 2026 under the peace deal. (Reporting by Luis Jaime Acosta Writing by Julia Symmes Cobb)
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