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MEXICO CITY, Sept. 18 (Reuters) - More than 20 inmates died and several dozen were wounded in riots in an overcrowded prison in the Mexican border city of Tijuana this week, police said on Thursday.

Violence first broke out on Sunday during family visiting hours in the La Mesa jail after a prisoner died inside, apparently abused by guards, a human rights inspector said.

Four inmates died in the clashes and another 19 were killed in new rioting on Wednesday. Police said the prisoners used firearms, but no security officials died in the clashes.

"They took advantage of visiting to start the riot. They went to the area of the prison where there are shovels and pick axes to break though the walls, they lit mattresses with cigarette lighters," said Carmen Salto, an investigator with Baja California's state human rights office.

The jail, built for around 3,000 people, was packed with more than 8,000, and family members claimed prisoners lived in dire conditions.

"The death (of the inmate) was the spark that pushed them over the edge, because we are talking about an unbelievable amount of overcrowding in buildings where they can't go out for 24 hours a day," said Salto, who visited the prison twice.

Video from the riots showed smoke billowing from the jail and inmates on the roof. Some waved white bedsheets while others threw rocks at heavily armed police officers. Salto said the prisoners did not have guns as police said.

Authorities regained control of the prison and transferred 200 dangerous inmates to other facilities, Baja California Gov. Jose Osuna said at a news conference.

La Mesa was notoriously corrupt before a 2002 police raid revealed some prisoners were living lavish lifestyles with contraband goods, brothels and even restaurants. After police retook control of the prison they destroyed that part of the jail, known as "the little town."

Tijuana, just south of San Diego, is a major corridor for drug trafficking and has seen violence soar in recent months even as President Felipe Calderon deployed thousands of troops to the city and across the country to crack down on warring cartels.

More than 2,700 people have been killed in drug violence this year in Mexico. (Reporting by Lizbeth Diaz and Mica Rosenberg, editing by Ross Colvin)

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