Masked gunmen forced residents onto the floor at El Camino a la Recuperacion center and then sprayed them with bullets, officials said. The center is located in the Buenos Aires Sur area of eastern Tijuana.
Officials have not announced a motive for the killings, but Mexico is gripped in a bloody war among drug cartels.
A total of 28,228 people have died in Mexico in drug-related war deaths, and Mexican President Felipe Calderon has ordered about 45,000 troops to 18 states where drug trafficking organizations are battling for access to the U.S. market.
Earlier this month in Ciudad Juarez, 14 people were gunned down at a family birthday party. The border city has become the epicenter of Mexico's drug war.
Just two weeks ago, President Felipe Calderon touted Tijuana as a success story in his nearly four-year-old drug war, noting during a festival to promote the city's industries that homicides are down from a peak in 2008.
Since his visit, drug gangs have resumed gruesome tactics not seen in the Tijuana for months, beheading rivals and hanging bodies from bridges. Some residents have expressed fear that the cartels are deliberating intensifying the violence to undermine Calderon's message.
The attack also comes about week after the government's record Oct. 18 seizure of 148 tons (134 metric tons) of marijuana in Tijuana.
While police have not identified the motive in the Tijuana slayings, drug gangs have attacked such centers before to target rival gang members.
Nationwide, more than 28,000 people have been killed in drug gang violence since December 2006, when Calderon deployed soldiers to battle the cartels in their strongholds in northern Mexico and along the Pacific coast.
While the government says most of the dead were involved in the drug trade, innocent bystanders have also died, like three people killed in the crossfire of a shootout between gunmen, police and soldiers in northern Coahuila state Sunday.
The victims were a 14-year-old boy and two women aged 18 and 47, according to a statement by the state prosecutors' office.
The statement said gunmen traveling in two vehicles opened fire on a convoy of federal police officers and soldiers in the city of Saltillo, Coahuila. The officers and soldiers returned fire.
It was not clear who fired the shots that killed the bystanders, but the state attorney general's office said it was investigating and expressed condolences to the victims' families.