The Eastern Bloc of the Revolutionary Armed Forces of Colombia, from September 2010 known as Bloque Comandante Jorge Briceño, in honour of the slain guerrilla leader, is considered by many to be the strongest military faction of the guerrilla group. It is divided into groups of 50-400 combatants in each group which patrol and control different areas of Colombia's Eastern and Central-Eastern territory, as well as helping to carry out the killing, taxation, and arrests necessary to advance the organization's financial and political goals. The specific divisions of the group are arguable. Some of the believed divisions or 'fronts', as they are commonly called, are shown below. Many of these fronts sometimes work together towards a certain mission, while others are further divided into 'columns' and 'companies' with a smaller number of members. For more general information see FARC Chain of Command.
Also known as the Guadalupe Salcedo Front, it is composed of up to 300 people. It operates mostly in the Arauca Department. Several police officers infiltrated one of the FARC’s 10th front camp in early 2008, preparing an attack on the camp by a police squadron. The attack came on July 29, killing 'Jurga Jurga', FARC commander and friend of FARC head ‘Mono Jojoy’. The ground offensive was follow by an airforce attack, killing 20 guerrillas, unofficial sources said. On October 22, 2011, members of the front killed 10 Colombians soldiers in an ambush in Arauca. On March 17, 2012, alleged members of the 10th front killed 11 members of the Colombian army. Security forces launched an offensive few days after the attack: on March 20, 3 alleged member of the FARC are killed and 4 captured by the army. On March 21, 33 FARC members were allegedly killed and 5 others captured.
Carlos Julio Ávila
Accused of executing three US citizens in 1999.
Killed by Colombian security forces on July 29, 2008 
Captured by Colombian security forces on April 11, 2009. Also second in command for the Reinel Mendez Company.
Captured on May 16, 2010, member of the Reinel Mendez Company.
Jhon Javier Ariza Gil
Also second in command for the Mobile Column Afonso Castellanos. Killed by Colombian security forces on August, 2010 
Killed by Colombian security forces on October 8, 2010, he had replaced "Milton".
María Johana Arias Jáuregui
Killed by Colombian security forces on October 8, 2010  She was lover of alias "Grannobles".
Alleged guerrilla girlfriend of 10th Front commander "Efren". Wounded and captured on July 2011.
One of the head front commander, captured on October 25.
Second front commander. Killed on March 21st, 2012.
Financial and logistical head of the front. Captured on August 18, 2012.
Alleged "Carracho" daughter. Medics of the front. Captured on August 18, 2012.
"David" and "Chocha Linda"
William Alexander Salazar Acosta
Front commander. Killed on September 23, 2012.
Also known as the Hermogenes Maza Front, it is composed of up to 130 people. It operates mostly in the Meta Department. As of 2011 this front operates in La Uribe, Vista Hermosa, La Macarena and other municipalities.
Also known as the Joaquin Ballen Front, it is composed of anything between 40 and 300 people. It operates mostly in the Vichada Department. Its current leadership is unclear. The front is, as of 2011, lead by El Cadete.
"Jacinto", "El Danto"
Killed in 2003.
Earlier he had belonged to the 16th Front. Killed on August 10, 2010.
Also known as the Jacobo Arenas Front, it is composed of up to 350 people. It operates mostly in the Meta Department. Its current leadership is unclear after a government offensive captured its two highest ranking militants.
"El Flaco Iván"
Rodrigo Alberto Salazar Montoya
Captured in 2005.
Gilberto de Jesús Jaramillo Arias
Captured in 2005.
Tocayo, El Negro
Demobilized in April 2010. (Son of John Freddy Balcázar alias “El Negro Antonio”)
Also known as the Jose A. Anzoategui Front, it is composed of up to 120 people. It operates mostly in the Meta Department and the Cundinamarca Department. Six members of the front are captured on March 25, 2012.
Also known as the Teófilo Forero Front, it is believed to be composed of around 150 militants although its urban numbers could be much greater. It operates mostly in the Cundinamarca Department and is considered the FARC's base in Bogotá. It is considered responsible for much of the terrorist activity that occurs in and around the capital.
Also known as the Yarí Front. Operates in the Meta Department. Three guerrillas from this front were killed in action in July 2011. On February 14, 2012, alias Dumar, his lover alias Gisella and two other rebels were killed by the army. Four other have been captured.
The following columns and companies also form part of the Eastern Bloc:
Compañía Marquetalia: Formed in January 2011. Operates in the Meta department. 54 members. According to a reportage in El Espectador the column's medics are treating the local population, providing basic health services where the state presence is weak.
Mobile Column Alfonso Castellanos: Faction of the 10th front, composed by up to 120 members. Operates in the Arauca Department. His leader, "Gabino", was killed on August 24, 2008 by the Colombian Military.  The second-in-command, John Javier Ariza Gil, alias "Milton Diaz, has been killed on August 19, 2010 .
Mobile Column Juan José Rondón: It is composed by up to 250 members and operates mostly in the Guaviare Department. Its last known leader, Octavio Salamanca, alias "Urias Cuéllar", was killed in 2001.
Special Forces: It is composed by up to 80 men and operates in what used to be the demilitarized zone.
Company Reinel Méndez: It is composed by up to 80 men and operates in what used to be the demilitarized zone.
Company Esteban Ramírez: It is composed by up to 80 men and operates in what used to be the demilitarized zone.
Company Manuela Beltrán: It is composed by up to 50 men and operates in the Cundinamarca Department. Its leader, Neftaly Murcia Vargas, known as “Camilo Tabaco”, was killed on September 3, 2008, according to the Colombian Army.
Company Abelardo Romero: It is composed by up to 40 men and operates in the Cundinamarca Department. On March 26, 2012, tens of member of the front are killed by an offensive of the army including the Company leader "Alonso Rivas". Yesid Borracho, the successor of Alonso Rivas, was killed on July 30.