Buenos Aires - An estimated 300,000 people gathered Tuesday in Buenos Aires in opposing rallies over the ongoing political confrontation over higher export tariffs for certain crops, local media reported.

The show of force - divided between a large pro-farmers' rally and a smaller pro-government event - came as the latest episode in a power struggle between conservative farmers' unions and the left-wing government over significant increases in export tariffs on soybeans and sunflower seeds. The row has already stretched for four months.

The Argentine Senate was expected to vote Wednesday on legislation to hike the tariffs, which was already passed by the lower House in the Argentine Congress.

Farmers' leaders have already vowed to continue demonstrating if the Senate votes for the tariff increases.

About 80,000 people gathered at the pro-government rally, the daily Clarin reported. The only speaker was former Argentine president Nestor Kirchner, husband of current President Cristina Fernandez de Kirchner.

He told supporters, including members of trade unions and other Peronist organizations, that the tariff increase was necessary for Argentina's growth.

A few kilometres away, farmers' leader Alfredo De Angeli complained before an estimated 220,000 people that the government was plundering the provinces.

Almost all of the Argentine opposition - which Fernandez de Kirchner defeated easily in October 2007 - have joined issue with the farmers.

The average tariff on soy was increased from 35 per cent to more than 43 per cent at current prices. The move was initially set to apply to almost all of the surplus, in case the price for soybeans were to rise above 600 dollars a tonne. But the government has since modified the proposal to set a tax ceiling and protect smaller producers.

Argentina is the third-largest producer of soybeans in the world, after the United States and Brazil. More than 95 per cent of its production is exported.

Farmers have prospered greatly in recent years, amid the high world prices for their produce and to the devaluation of the peso. (dpa)