BUENOS AIRES, March 5 (Reuters) - U.S. foe Venezuelan President Hugo Chavez will lead a leftist rally in an Argentine stadium on Friday, right when U.S. President George W. Bush is due to arrive in neighboring Uruguay for a state visit.

Dozens of Argentine activist groups are planning the event in the Ferrocarril Oeste soccer stadium, which can hold up to 40,000 people, organizers said on Monday.

Bush's visit is part of a regional tour in which he will travel to Brazil, Uruguay, Colombia, Guatemala and Mexico.

Jorge Ceballos, an Argentine activist who is organizing the rally, told local radio it is meant as an "alternative message" to U.S. policies in the region.

"The United States is trying to block the expansion of Mercosur," said Ceballos, who also serves as deputy secretary in Argentina's Social Development Ministry.

Venezuela became a member of the Mercosur trade bloc last year, joining Argentina, Brazil, Paraguay and Uruguay. Smaller nations Paraguay and Uruguay complain they do not benefit from the bloc and seek permission to make bilateral trade deals.

Bush's visit to Uruguay has stoked speculation that the South American country aims to strike a free-trade deal with the United States, outside Mercosur.

Uruguayan President Tabare Vazquez reaffirmed his country's commitment to Mercosur last week after Brazilian President Luiz Inacio Lula da Silva visited Uruguay, but he also reiterated demands for fairer treatment within the bloc.

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