* Mexican police capture violent trafficker "La Barbie"
* Still unclear whether drug lord will be sent to U.S.
* Eight die in attack in tourist spot Cancun (Adds comment on possible extradition)
By Missy Ryan and Cyntia Barrera
MEXICO CITY, Aug 31 (Reuters) - Mexico paraded one of its most violent drug lords on Tuesday after a police raid that President Felipe Calderon's government hopes will mark a breakthrough in its campaign against powerful cartels.
But the capture of Edgar "La Barbie" Valdez, a Texas-born 37-year-old, may do little to halt the flow of drugs into the United States or staunch bloodshed in Mexico's most violent areas, many of them along the U.S. border.
In a sign of the widening violence, eight people were killed in the Caribbean resort of Cancun early on Tuesday when suspected drug hitmen threw Molotov cocktails into a bar on the city's outskirts, the local attorney general's office said.
In Mexico City, masked police paraded a handcuffed Valdez before reporters. Wearing a green polo shirt and jeans, the man nicknamed "La Barbie" for his fair complexion grinned openly as officials discussed his capture near Mexico City on Monday.
"This operation closes a chapter in drug trafficking in Mexico," senior federal police official Facundo Rosas told local television.
Six other men, including another Texan, were arrested with Valdez and police found weapons, vehicles, cocaine and cellphones at a safe house guarded by cartel gunmen.
Washington declined to say whether it would push for Valdez to be sent to face trial in U.S. courts where he has been indicted for drug trafficking.
"The federal police plan is to first process him here in Mexico for alleged crimes ... and then there are the cases pending outside the country, especially the United States," said Rosas.
Profile of Valdez [ID:nN30202980]
Full coverage of the drugs war [ID:nMEXDRUGS]
Political risks in Mexico [ID:nRISKMX]
FACTBOX on latest attacks [ID:nN29121638]
Yet the arrest is unlikely to end the bloodshed that presents a growing image problem for Mexico as it struggles out of recession and seeks to hold on to tourist revenues.
More than 28,000 people have died since Calderon launched his crackdown in late 2006. The violence shows no sign of stopping as gangs battle for control of smuggling routes.
Officials say Valdez, as a leader of the Beltran Leyva cartel based in central Mexico, trafficked a tonne of cocaine each month and was responsible for "several dozen" murders.
He is believed to be behind beheadings of rivals, the torture and mutilation of victims and the slaughter of the family of a marine who took part in the killing of his former boss Arturo Beltran Leyva in December.
But Valdez's operations were small compared to Mexico's top gangs -- the Sinaloa, Gulf and Juarez cartels -- that smuggle the majority of the 140 tonnes of cocaine the United Nations estimates that Mexico exports to the United States every year.
Neither is the arrest likely to end violence in border areas like Ciudad Juarez, across from El Paso, Texas, or in Mexico's wealthy northern city of Monterrey, which is being sucked into the drug war this year.
Valdez's arrest follows an operation in July that killed Ignacio "Nacho" Coronel, No. 3 in the Sinaloa cartel.
While the government hopes the victories will weaken Mexican cartels, such operations have intensified bloodshed at least temporarily as subordinates battle for control of gangs believed to rake in up to $40 billion a year.
Valdez had been a top contender to head the Beltran Leyva cartel since its boss was killed by soldiers in December.
"The investigation has not been concluded ... and at this stage it is not clear who could replace him," Rosas said.
Born into a middle-class family, Valdez is said to have played American football at school and developed a taste for luxury before coming to Mexico to work in the drug trade. (Additional reporting by Andrew Quinn and Jeremy Pelofsky in Washington, Robin Emmott in Monterrey and Carlos Calvo in Mexico City; writing by Robin Emmott and Missy Ryan; Editing by John O'Callaghan)