BRASILIA, April 13 (Reuters) - Landless peasants torched at least 30 tonnes of unplanted sugar cane this week in Brazil's main ethanol-producing state, pledging to invade other farms they say are illegally leasing land to producers of the alternative fuel, a peasant group said on Friday.

Lourival Placido de Paula, a leader of the Landless Peasants' Movement, or MST, in western Sao Paulo state, said the movement would "continue the struggle" against a boom in new plantings of sugar cane, which is used to make ethanol.

De Paula said farms all around the region were planting or preparing to plant cane.

Ethanol is starting to come under fire from left-leaning critics, who say expanding cane fields in Brazil could crowd out small farmers and food crops.

The issue is adding a new dimension to an age-old struggle for land in Brazil, where millions live in poverty but where nearly half of the arable land is owned by 1 percent of the population, according to official data.

The MST follows a strategy of organizing peasants to invade and settle land they say has been promised by the government.

The government has planned to redistribute property in some areas as part of a land reform program, but legal disputes have delayed the issuing of titles.

Traditionally the MST has occupied unproductive land, but in recent years it has also targeted plantations owned by large forestry companies and agribusiness interests.

The shift is straining the MST's relations with the left-leaning government of President Luiz Inacio Lula da Silva, who signed an ethanol promotion accord with U.S. President George W. Bush last month and is actively backing biofuels as a way to boost economic activity.

Some Latin American allies on the left, including Venezuelan President Hugo Chavez, have spoken out against ethanol. Cuban President Fidel Castro has penned editorials saying ethanol will take land away from crops for food.

De Paula said MST peasants invaded about 6,000 hectares (14,826 acres) around the town of Andradinha this week, half of which he said property owners had illegally leased to cane distillers in an attempt to inflate the value of the land.

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