Buenos Aires: A three-month standoff between Argentina's government and farmers over a tax hike turned violent on Saturday when military police in riot gear used batons to try to clear roadblocks on a main highway.

Farmers began their fourth strike in three months to repudiate the government crackdown. The protest will run through Wednesday, with farmers halting sales of grains and most agricultural products.

"Today was incredible. After what happened in Gualeguaychu, the situation exploded and thousands of farmers mobilised," said Eduardo Buzzi, head of the Argentine Agrarian Federation, one of the four farm groups leading protests.

Military police dragged demonstrators off Route 14 near Gualeguaychu, in Entre Rios province, to allow trucks carrying food and other goods to pass amid growing shortages.

Hundreds of townspeople rallied around the farmers and forced the police to retreat.

Police also briefly detained a leading figure of the agrarian federation, sparking more road blocks by farmers.