Factbox - Worst atrocities in Mexico's drug war

(Reuters) - A fight between rival gangs at a prison just outside Monterrey in northern Mexico left 44 dead on Sunday, state government officials said.

Below are some of the worst attacks since President Felipe Calderon took office in December 2006 and declared war on powerful drug cartels. About 50,000 people have been killed in drug-related violence since then.

* Sept 15, 2008 - Suspected members of the Zetas drug gang toss grenades into a crowd celebrating Mexico's independence day in the western city of Morelia, killing eight people and wounding more than 100.

* Jan 31, 2010 - Suspected cartel assailants kill 13 high school students and two adults at a party in Ciudad Juarez across from El Paso, Texas.

* March 13 - Hitmen kill three people linked to the U.S. Consulate in Ciudad Juarez, provoking "outrage" from U.S. President Barack Obama.

* June 28 - Suspected cartel gunmen shoot and kill a popular gubernatorial candidate in the northern state of Tamaulipas in the worst cartel attack on a politician to date. Rodolfo Torre, 46, and four aides from the opposition Institutional Revolutionary Party, or PRI, were ambushed on their way to a campaign event for the July 4 state election.

* July 18 - Gunmen burst into a birthday party in the northern city of Torreon, using automatic weapons to kill 17 party-goers and wound 18 others. Mexican authorities said later those responsible were incarcerated cartel hitmen let out of jail by corrupt officials. The killers allegedly borrowed weapons and vehicles from prison guards and later returned to their cells.

* July 24 - Police unearth 51 bodies in a grave outside Mexico's business capital, Monterrey, in northern Mexico over several days. Some corpses were burned beyond recognition.

* Aug 25 - Marines find the bodies of 58 men and 14 women at a ranch near the Gulf of Mexico in Tamaulipas state, 90 miles (150 km) from the Texas border, after a firefight with drug gang members in which three gunmen and a marine died.

* April 2011 - Officials unearth the first of what turn out to be more than 450 bodies buried in mass graves in the northern states of Durango and Tamaulipas.

* Aug 20 - Five headless bodies are found in Acapulco, taking the number of people killed in the popular Pacific resort to at least 25 in one week.

* Aug 25 - Masked gunmen torch a casino in Monterrey, killing 52 people, most of them women. The attack takes less than three minutes.

* Sept 20 - Thirty-five bodies are found abandoned in two trucks on an underpass in the eastern Gulf city of Veracruz, which had been largely untouched by the violence.

* Oct 6 - Mexican security forces find 32 bodies at several locations around Veracruz, just two days after the government unveils a plan to bolster security in Veracruz state.

* Nov 24 - More than 20 bodies are found in cars in Mexico's second biggest city, Guadalajara, a day after the burned bodies of 16 people are found in the home state of the country's powerful drug lord, Joaquin "Shorty" Guzman.

(Editing by Peter Cooney)