MEXICO CITY, Aug 25 (Reuters) - Armed gangs blocked highways throughout Guadalajara, Mexico's second biggest city, on Saturday and vehicles were set on fire amid a surge in drug-war violence.

Police confirmed seven unauthorized roadblocks constructed with charred, smoldering cars and trucks within the Guadalajara city limits and 15 others in the surrounding Jalisco state, in western Mexico.

Luis Carlos Najera, police chief for Jalisco state, told reporters at a news conference late Saturday that one man was seriously wounded by gunfire, but no arrests had been made.

"We don't know who is behind this operation," said Najera.

He added that all of the roadblocks have since been cleared.

Guadalajara has not seen the level of drug cartel-related violence as other areas, but similar flaming roadblocks shook the city in early March as military forces successfully sought to arrest a prominent cartel leader.

Two people were killed during that operation.

Jalisco state has been a stronghold of the Sinaloa Cartel since the 1980s, when traffickers first started to use Mexico as a "trampoline" to bounce cocaine into the United States.

It has recently seen an upsurge in violence as Sinaloa Cartel gunmen battle rivals from the upstart Zetas cartel, which is displacing older trafficking groups in many parts of Mexico.

More than 55,000 Mexicans have been killed in drug-related violence since President Felipe Calderon took office in December 2006 and launched his crackdown on cartels. (Reporting By David Alire Garcia; editing by Todd Eastham)

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