Tijuana's police chief was fired Monday following three days of violence that left 37 people dead in this border city plagued by warring drug gangs, including nine men found decapitated and four children caught in shootouts.

Tijuana Public Safety Secretary Alberto Capella has been replaced by his second-in-command, army Cmdr. Julian Leyzaola, according to statement from the office of Mayor Jorge Ramos.

No reason was given for Capella's abrupt dismissal, although it followed a particularly bloody weekend in the city across the border from San Diego.

Three police officers were among the nine decapitated men, whose bodies and heads were discovered Sunday in a poor Tijuana neighborhood, said Baja California state Attorney General Rommel Moreno. Their police credentials were stuffed in their mouths.

More than 200 people have been killed in the past month in Tijuana, where officials say rival cells of the Arellano-Felix drug cartel have been waging a bloody battle.

Capella had promised to work to restore public trust in Tijuana's police force, insisting that corrupt or abusive officers were being prosecuted.

Officers in the city are so mistrusted that the army once invited citizens to report crimes to soldiers instead of to police. For a time last year, federal authorities took guns away from the city police.

Police were investigating whether some of the 37 deaths between Saturday and Monday were part of a retaliatory spree sparked by the killing of a 25-year-old woman believed to be a drug trafficker's girlfriend, said Baja California state Attorney General Rommel Moreno.

He said interviews with families members indicated that 80 percent of the victims had been involved in drug dealing.

But four of the dead were children.

Two brothers, aged 4 and 13, had been waiting for their parents outside a convenience store when gunmen opened fire, killing the boys and several adults. A 14-year-old boy working at locksmith's kiosk was shot dead in an attack on a neighboring business. And a 12-year-old was killed when the car he was riding was sprayed with bullets.

Violence has soared in Mexico as drug cartels compete for smuggling routes and battle government forces.

In the northwestern state of Sonora, a 15-year-old boy was found shot to death under a tree. Police named no suspects or motive.

In southern Michoacan state, gunmen burst into the offices of a local cattle ranchers' association and killed one of its directors, a 27-year-old woman. Police arrested two suspects but gave no possible motive.

Mexican newspapers have reported that more than 4,000 people have been killed across the country this year in drug-related violence.

The federal government does not regularly release homicide figures, although officials have acknowledged that killings have surged in the last two years.

Since taking office in 2006, President Felipe Calderon has sent more than 20,000 soldiers across the country to root out cartels, a crackdown that is popular among many Mexicans.