Bolivian President Evo Morales has ordered the nationalisation of four private electricity companies. Police moved into the offices of Corani, Valle Hermoso and Guaracachi firms, following Mr Morales' decree.
Corani is half owned by a subsidiary of France's GDF Suez. Guaracachi's main partner is Britain's Rurelec, while ELFEC and Valle Hermoso are local.
President Morales said that the government now controlled 80% of electricity generation in the country.
Last year, Mr Morales announced the takeover of a subsidiary of British oil company BP, which supplied jet fuel across the Andean nation.
He has recently nationalised oil and gas reserves to redistribute wealth to Bolivia's indigenous majority.
The four companies seized on Saturday account for more than half of Bolivia's electricity market.
They emerged following the privatisation of the state National Electricity Company in the 1990s.
President Morales nationalised the gas and oil industry in 2006 and the national telephone company in 2008.
Both those announcements also came on May Day, says the BBC's Andres Schipani in Bolivia.
Since then he has taken control of several utility and commodities companies.
