BOGOTA, Feb 19 (Reuters) - About 2,000 Colombian peasants clashed with police on Tuesday in a swelling protest over a government program to eradicate their coca leaf crops used to manufacture illegal cocaine.

Protesters, some throwing rocks, smashed a toll booth and blocked a major highway leading to the Caribbean coast from the northwestern city of Medellin to demonstrate against a campaign to send crews of workers to dig up the coca leaf plants.

The clashes illustrated the task President Alvaro Uribe faces to wipe out drug crops even with the help of billions of dollars in U.S. military aid to combat traffickers and leftist rebels fighting Latin America's oldest insurgency.

Authorities were trying to negotiate with farmers demanding two years to shift to legal crops, even though the government has promised to push ahead with a plan to eradicate 247,000 acres (100,000 hectares) of coca by hand nationwide this year.

"They are asking for solutions to their food security and sustenance," the mayor of the town of Taraza, Miguel Angel Gomez, told Reuters by telephone from Antioquia province.

Last year, Colombia announced it would reduce fumigation spraying of coca areas and begin intensive manual eradication, which analysts say is more effective and less damaging to other legal crops.

The government says it plans to step up social programs offering alternatives for coca farmers. Critics say the state presence is still weak in areas where coca is grown.

"We're protesting because if they finish off the illegal crops, which we all know are illegal and damaging, then they finish off our way of sustaining our families," one farmer told local television.

Authorities have blamed the Revolutionary Armed Forces of Colombia, or FARC, guerrillas for fomenting the protests.

Weakened by Uribe's U.S.-backed security campaign, the FARC is still potent in some rural areas. The rebels are labeled a cocaine-smuggling terrorist group by the United States and the European Union.

Colombia remains the world's No. 1 cocaine producer with about 600 tonnes of the drug leaving the Andean country each year for the United States and Europe through Central America, Venezuela, Africa and the Caribbean islands. (Reporting by Luis Jaime Acosta; Writing by Patrick Markey)

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