LIMA, May 1 (Xinhua) -- The Bolivian government headed by President Evo Morales faces a tough challenge as an autonomy referendum proposed by the Santa Cruz provincial government, possibly the first in a row, will take place Sunday.

    The provincial government's push for autonomy will be followed by three other natural gas-producing regions -- Tarija, Beni and Pando. The four autonomy-seeking territories altogether account for 35 percent of the country's 9.5 million population, and 65 percent of its gross domestic product.

    The other five Bolivian provinces -- La Paz, Oruro, Chuquisaca, Cochabamba and Potosi -- do not support Santa Cruz's autonomist move.

    Morales has said he views the May 4 referendum as an illegal separatist bid, and has vowed to ignore any autonomy declaration.

    His government on Thursday froze the accounts of the province of Santa Cruz, aggravating tensions between Santa Cruz's opposition governor and the central government.

    Santa Cruz's governor Ruben Costas said Sunday's autonomous referendum seeks the birth of a "second Bolivian Republic."

    "We are tired of a centralism where (the government members) do whatever they like," said Costas.

    He said he believes "there will be a domino effect" after his province took the first step.

    According to polls, 75 percent of Santa Cruz's voters would vote "YES" for autonomy.

    Santa Cruz's referendum is to be followed by three other regions in June.

    Morales' government said that the outcome of Santa Cruz's autonomy vote will be invalid since it is ratified neither by Congress nor the National Electoral Court.

    The crisis was triggered by the president's plans to overhaul Bolivia's constitution to redistribute much of the wealth of the eastern provinces to the poorer Andean highlands.

    The conflict has taken on an ethnic context, pitting the poor indigenous majority of the mountainous area against the rich in the lower provinces.

    Bolivia's fight for integrity was strongly supported by multinational groups in Latin America.

    The Rio Group said it rejects "any attempt to interrupt the democratic process" in Bolivia.

    The Organization of American States (OAS) has sent a mission to Bolivia trying to ease the conflict between the opposition and Morales' government.

    The group also denied that it will send observers to the autonomy referendum in Bolivia's Santa Cruz province on May 4.

    Bolivia's Interior Vice Minister Ruben Gamarra said the government will reinforce police presence in Santa Cruz on Sunday to ward off conflicts between opponents and sympathizers of Morales.

    The government also accused the U.S. ambassador to Bolivia, Philip Goldberg, of siding with the rebel provinces.

    "Ambassador Philip Goldberg has unveiled an agenda more political than diplomatic in Bolivia, and this agenda is linked to opponents of the current government," Interior Minister Alfredo Rada said.

    The U.S. embassy responded with a statement saying it supported the "unity and democracy" of Bolivia.

    Venezuelan President Hugo Chavez, a close ally of Morales, hastily called a summit earlier this week with Morales, Nicaraguan President Daniel Ortega and Cuban Vice President Carlos Lage to make a joint declaration of support for the Bolivian leader.